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Norman E. Rosenthal, Dan A. Oren: Chapter 28. Light Therapy, in Gabbard’s Treatments of Psychiatric Disorders, 4th

Edition. Edited by Glen O. Gabbard. Copyright ©2009 American Psychiatric Publishing, Inc. DOI:

10.1176/appi.books.9781585622986.257853. Printed 5/10/2009 from www.psychiatryonline.com

Gabbard’s Treatments of Psychiatric Disorders > Part V. Mood Disorders >

Chapter 28. Light Therapy

INTRODUCTION

Although exposure to environmental light has been regarded as therapeutic since ancient times and

artificial light has been recommended for a variety of conditions for more than 100 years, the use of

visible light for a narrow and specific range of psychiatric conditions has been advocated only in the

past 25 years. For a historical perspective, the interested reader is referred elsewhere (Rosenthal

2005; Wehr and Rosenthal (1989).

Light therapy in its modern form has usually been administered by means of light boxes, and most

research has involved the use of such devices, although newer devices may prove useful. Our focus

in this chapter is on the modern use of light therapy.

DIAGNOSTIC INDICATIONS AND CONTRAINDICATIONS

Indications

The chief indication for light therapy is seasonal affective disorder (SAD) (Rosenthal et al. 1984) or,

as classified in DSM-IV (American Psychiatric Association 1994), any form of recurrent mood

disorder, with depressive episodes in winter, to which the specifier “with seasonal pattern” applies

(Table 28–1). The evidence for the efficacy of light therapy in SAD and other conditions is

summarized in Table 28–2.

Table 28–1. DSM-IV criteria for seasonal pattern specifier

Specify if:

With seasonal pattern (can be applied to the pattern of major depressive episodes in bipolar I disorder,

bipolar II disorder, or major depressive disorder, recurrent)

1. There has been a regular temporal relationship between the onset of major depressive episodes in bipolar I

or bipolar II disorder or major depressive disorder, recurrent, and a particular time of the year (e.g., regular

appearance of the major depressive episode in the fall or winter).

Note: Do not include cases in which there is an obvious effect of seasonal-related psychosocial stressors

(e.g., regularly being unemployed every winter).

1. Full remissions (or a change from depression to mania or hypomania) also occur at a characteristic time of

the year (e.g., depression disappears in the spring).

1. In the past 2 years, two major depressive episodes have occurred that demonstrate the temporal seasonal

relationships defined in criteria A and B, and no nonseasonal major depressive episodes have occurred during

that same period.

1. Seasonal major depressive episodes (as described above) substantially outnumber the nonseasonal major

depressive episodes that may have occurred over the individual’s lifetime.

Source. Reprinted from American Psychiatric Association: Diagnostic and Statistical Manual of Mental

Disorders, 4th Edition, Text Revision. Washington, DC, American Psychiatric Association, 2000, p. 427.

Copyright 2000, American Psychiatric Association. Used with permission.

Table 28–2. Indications for light therapyPrint: Chapter 28. Light Therapy

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Condition Evidence for efficacy of light therapy

Seasonal affective

disorder (SAD)

American Psychiatric Association (APA)–sponsored meta-analysis found light therapy

for SAD to have efficacy comparable to that of antidepressant medications for

nonseasonal depression (Golden et al. 2005)

Subsyndromal SAD One controlled study (Kasper et al. 1989); clinical impressions

Nonseasonal

depression

Light therapy is effective, according to APA-sponsored meta-analysis (Golden et al.

2005)

Premenstrual dysphoric

disorder (PMDD)

A few controlled studies (Lam et al. 1999; Parry 1998); probably effective

Bulimia nervosa Two controlled studies (Blouin et al. 1996; Lam et al. 1994); probably effective;

better response in bulimia with winter exacerbations

Delayed sleep-phase

syndrome

One controlled study (Rosenthal et al. 1990)

Early morning

awakening

One controlled study (Lack and Wright 1993)

Insomnia in the elderly Two controlled studies (Campbell 1998; Campbell et al. 1993)

Jet lag Case reports and clinical impressions (Boulos 1998; Oren et al. 1993)

As can be seen in Table 28–2, SAD is the only condition for which the indication for light therapy

rests on a large body of scientific evidence (discussed later in this chapter), although a recent

American Psychiatric Association–sponsored meta-analysis (Golden et al. 2005) found light therapy

to be effective for nonseasonal depression as well and it is probably also effective for bulimia

(Blouin et al. 1996; Lam et al. 1994), premenstrual dysphoric disorder (PMDD) (Lam et al. 1999;

Parry 1998), and circadian sleep disturbances (Boulos 1998; Oren and Terman 1998; Rosenthal et

1. 1990). Subsyndromal SAD, also known as the “winter blues,” has been found to respond to light

therapy in one controlled study (Kasper et al. 1989)—a predictable finding given the resemblance of

this condition to its more severe syndromal counterpart.

Light therapy also appears to be of value in nonseasonal depression, though there are fewer

high-quality studies on which to base this conclusion (Baumgartner et al. 1996; Kripke 1998; Voltz

et al. 1991). Patients with antepartum depression may also benefit from light therapy, as evidenced

by both open-label and controlled studies (Epperson et al. 2004; Oren et al. 2002). According to the

American Psychiatric Association meta-analysis, there is insufficient evidence to support the

efficacy of light therapy as an adjunctive treatment in nonseasonal depression, but it seems likely

that light therapy would be effective if used this way, given that it works as a solo treatment for

patients with depression. Clinical experience indicates that many patients with dysthymia and other

mood disorders who experience winter exacerbations but do not meet criteria for SAD may

nonetheless benefit from light therapy. Although not definitively established, the value of light

therapy for PMDD is highly likely, given a few positive controlled studies (Lam et al. 1999; Parry

1998; Parry et al. 1989). It remains to be established whether PMDD patients with winter

exacerbations do especially well with light therapy.

The capacity of light to shift circadian rhythms in humans is very well accepted, presumably

because of the extensive literature on similar effects in animals (Pittendrigh 1989; Winfree 1987).

There is general agreement that such phase-shifting properties might be helpful to persons with

circadian rhythm disturbances, and debate centers around the details of how such phase shifts can

best be accomplished. Patients with delayed sleep-phase syndrome, a condition in which there is an

inability to fall asleep and to wake up at conventional times, respond favorably to a combination of

bright light exposure in the morning and light restriction in the evening (Rosenthal et al. 1990).

Likewise, patients with advanced sleep-phase disorder, a condition associated with early

awakening and early sleep times that tends to afflict older persons, may respond favorably to bright

light in the evening hours (Lack and Wright 1993). Such a treatment regimen may be generallyPrint: Chapter 28. Light Therapy

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beneficial for insomnia in elderly patients (Campbell et al. 1993).

The effects of light exposure or restriction on human circadian rhythms can be predicted to some

degree by the so-called phase–response curve (PRC), which is a graphic representation of the

relationship between these parameters. A PRC appears to be a universal property of biological

organisms (Winfree 1987), including humans (Czeisler et al. 1990; Minors et al. 1991). The human

PRC predicts that in most persons living according to a conventional sleep–wake schedule, light

exposure at the beginning of the night will shift rhythms later and light exposure at the end of the

night will shift rhythms earlier. Light exposure in the middle of the day, the dead zone, will have

minimal effects on circadian rhythms. Although it is relatively easy to predict when to administer

light to patients with stable disorders of circadian rhythms, as in the case of patients with delayed

or advanced sleep-phase syndrome, it is more difficult to do so when circadian rhythms are in flux,

for example, after travel across several time zones, which results in jet lag, or in rotating shift

work. Guidelines as to how precisely timed light exposure (and light restriction) can be used to

combat the lethargy, fatigue, insomnia, and cognitive disturbances that may accompany such

circadian rhythm disturbances go beyond the scope of this chapter but can be located elsewhere

(Eastman 1992; Oren et al. 1993).

Contraindications

There are no absolute contraindications to light therapy, although there are some circumstances in

which caution is required. These include when the patient 1) has a condition that might render the

eyes more vulnerable to phototoxicity, 2) has a tendency toward mania, 3) has a photosensitive

skin condition, or 4) is taking a photosensitizing medication or herb (such as St. John’s wort or a

psoralen).

Although there have been no documented cases in which light therapy, when properly administered

and supervised, has caused damage to the eyes, the potential for such damage, particularly from

blue wavelengths, has been raised as a theoretical possibility (M. Terman et al. 1990). The

likelihood of light-induced damage to the retina might be increased in patients with retinal diseases

such as macular degeneration or retinitis pigmentosa. Therefore, patients with retinal abnormalities

should not be given light therapy without careful prior evaluation and ongoing supervision by an

ophthalmologist. The degree to which patients with normal eyes should have their eyes regularly

monitored during the course of light therapy has been debated, with some advocating more

stringent (Roberts et al. 1992; M. Terman et al. 1990; Vanselow et al. 1991) and others advocating

less stringent (Waxler et al. 1992) monitoring. Concomitant use of certain antidepressants or other

medications that may enhance phototoxicity has been cited by some investigators as a reason for

additional caution and more frequent monitoring (M. Terman et al. 1990), although no increased

prevalence of eye damage has been reported in persons on such medications, despite regular

exposure to bright sunlight. In addition, there is no evidence of any increased prevalence of eye

problems in patients who have received light therapy for several years, a proportion of whom have

taken concomitant medications (Schwartz et al. 1996).

Although light therapy can occasionally induce hypomania or, very rarely, mania, a history of mania

is not in itself an absolute contraindication to treatment. The likelihood of hypomanic symptoms can

be greatly decreased by warning vulnerable patients about this possibility ahead of time, being

vigilant about the patient’s clinical condition, and titrating the dosage of light treatment so as to

minimize or counteract the development of hypomanic symptoms. For those with a history of

mania, it is prudent to start treatment with dosage durations of as little as 5 minutes per day and to

be especially vigilant.

As ultraviolet (UV) light is not thought necessary for the antidepressant or circadian-rhythm

shifting effects of light therapy and is clearly a risk factor for toxic effects, physicians should be

careful to prescribe light boxes that screen out most (if not all) UV rays (Oren et al. 1990). UV light,

which may cause facial erythema similar to that seen with suntanning, may in some cases reach the

user. This effect may cause problems for patients with systemic lupus erythematosus (SLE) or other

photosensitive skin conditions. Nevertheless, with the help of a sun-blocking cream, a few patientsPrint: Chapter 28. Light Therapy

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with SLE have been effectively treated with light therapy without induction of facial erythema

(Moul 1992). This would suggest that SLE is not necessarily a contraindication to the use of light

therapy. Neither is a history of skin cancer an absolute contraindication, although when such a

history is present, as with SLE and other photosensitive skin conditions, if light therapy is used,

every attempt should be made to screen out UV radiation. Precautions include the use of specially

coated fluorescent bulbs, special UV filters on the inside of the diffusing screen, and commercial

sunscreen ointments (which absorb about 95% of incoming ultraviolet B light). The value of

sun-blocking ointments has been called into question, however, by a study that found that the use

of such ointments was of no value in preventing UV-induced melanomas in mice, despite the

capacity of these ointments to inhibit the development of erythema (Wolf et al. 1994). The

relevance of this study to the use of light therapy in persons who have previously had skin cancer is

unclear at this time, but those with such vulnerability should be counseled to be attentive to any

skin changes in light-exposed areas and to report such changes promptly to their doctor.

SIDE EFFECTS

Light therapy is generally well tolerated, and side effects, when they occur, are usually mild and

short-lived and can often be managed by decreasing the initial exposure to light. This can be

accomplished by shortening treatment sessions, having the patient sit a little farther from the light

box, or decreasing glare (e.g., by tilting the light box so that the upper edge is angled toward the

patient).

Common side effects include irritability, headache, nausea, hypomania, and eye strain (Rosenthal

et al. 1984; M. Terman and Terman 1999; Wirz-Justice et al. 1986). Interestingly, the Termans

found that mild nausea predicts a favorable response to light therapy.

Serious adverse events have rarely been documented in the course of light therapy. These include a

few patients who became manic following light treatment, two of whom were patients with unipolar

nonseasonal depression (Schwitzer et al. 1990). Suicidal ideation or attempts have been described

in three patients within the first 2 weeks of starting light therapy (Praschak-Rieder et al. 1997),

and one other patient committed suicide after 5 days of light treatment (Haffmans et al. 1998).

Overall, light therapy appears to reduce the severity of suicidal ideation, as evidenced by a

retrospective analysis by Lam et al. (2000) of 191 patients with SAD. These researchers found an

improvement in suicide ratings in 45% of patients and a worsening in 3%. It is unclear at this time

whether such worsening of suicidal ideation or risk is a function of the underlying illness or

whether it may actually be aggravated by light therapy in selected cases, just as has been

suggested for antidepressant medications (Fergusson et al. 2005). Regardless of the exact etiology,

such reports should heighten the awareness among clinicians about the potential for exacerbating

suicidal tendencies, particularly in the early phases of treatment, and encourage close clinical

observation of treatment response as with any other form of treatment.

In summary, side effects of light therapy are generally mild and easily managed and few patients

drop out of treatment on that account. Serious side effects are rare. A link between light therapy

and suicidal tendencies has not clearly been established. Overall, a cost–benefit analysis clearly

favors the use of light therapy in SAD and, with slightly less support, in patients with nonseasonal

depression.

MAINTENANCE/CONTINUATION THERAPY

While there is compelling evidence in support of the short-term efficacy of light therapy, to date

there have been no studies of the long-term efficacy of light therapy for SAD. Indeed, the concept

of long-term studies for this condition is problematic because the condition is by definition

self-limiting. Nevertheless, it would be useful to know whether light therapy works for the duration

of the winter, year after year. In the absence of properly controlled studies, we must rely on clinical

experience and systematic retrospective evaluation of SAD patients to guide us in our appraisal of

the long-term efficacy of light therapy.

In a retrospective follow-up of 59 SAD patients, 24 of 57 who had originally been treated with lightPrint: Chapter 28. Light Therapy

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therapy (42%) continued to use light therapy regularly over the duration of follow-up (an average

of 4 years), and all of these patients regarded light therapy as highly effective, with most reporting

undiminished efficacy over time (Schwartz et al. 1996). Of the 33 patients who stopped using light

therapy at some point during the follow-up period, 16 reported that light therapy had decreased in

its effectiveness over time, 9 reported that light therapy was too inconvenient to continue, and 8

reported that they felt they had not become sufficiently depressed during subsequent winters to

warrant renewed treatment. These data suggest that a significant proportion of SAD patients will

benefit from long-term treatment.

Clinical experience suggests that patients who respond once to light therapy will do so on

subsequent occasions as well and that light therapy continues to work over time. In cases in which

light therapy does appear to become less effective over time (a problem not unfamiliar in

psychopharmacology), it is generally difficult to determine whether such a decrease in

effectiveness reflects tolerance or deepening of the depression. The commonly observed

exacerbation of symptoms after withdrawal of light therapy suggests that light therapy does indeed

provide ongoing antidepressant benefits. Some investigators have suggested that light therapy

administered early in the winter is prophylactic against the development of SAD symptoms later in

the season, even if treatment is discontinued (Meesters et al. 1991, 1993). Others, disputing this

claim, have found that patients develop symptoms as usual if treatment is discontinued (J. S.

Terman et al. 1994).

FORMAL ASPECTS OF LIGHT THERAPY

The beneficial effects of light therapy appear to be mediated via the eyes rather than the skin

(Wehr et al. 1987). Clinicians should explain this finding to patients, who may not otherwise

appreciate the need for the eyes to be open during treatment sessions and at the appropriate

distance from the light source. In addition, it is important to emphasize that there is scant evidence

that tanning salons, where the eyes are generally covered and the subject’s skin is exposed to light,

are useful for SAD. In addition, the light sources in tanning salons are by definition high in UV rays,

which can be harmful to both the eyes and the skin. For these reasons and also because UV light is

not necessary for an antidepressant effect (Lam et al. 1991; Oren et al. 1991), devices that emit UV

light are contraindicated.

Although the initial light treatment studies were performed with “full-spectrum” lights (i.e., lights

that mimic, in their wavelength range and emission, the spectrum of sunlight), more recent studies

have shown ordinary fluorescent lights to have significant antidepressant effects (Oren and

Rosenthal 1992; Rosenthal 1993). There is no evidence that full-spectrum light is superior to

ordinary fluorescent light (Lam et al. 1992). Patients often ask whether it would be helpful for them

to replace all the fluorescent lights in their home or workplace with full-spectrum lights, indicating

a lack of understanding that it is the intensity of light rather than its spectrum that appears to be

most critical for achieving an antidepressant effect.

Initially, white light of 2,500-lux illuminance (a measure of perceived brightness at the eyes) was

tested as the active treatment modality against light sources of lower intensity (Oren and

Rosenthal 1992; M. Terman and Terman 1992) and was found to be superior. The 2,500-lux

illuminance was chosen because it had previously been found to be capable of suppressing

nocturnal melatonin secretion (Lewy et al. 1980). The original 2,500-lux light boxes were placed

vertically on a table or work surface 3 feet away from the user’s eyes. Many currently marketed

light boxes are slanted toward the user’s face in such a way as to bring 10,000 lux to the eyes and

reduce glare. The chief advantage of the brighter new boxes is that antidepressant effects can be

obtained from shorter treatment sessions (e.g., 30 minutes rather than 2 hours) in some patients

(J. S. Terman et al. 1990), suggesting a reciprocal relationship between duration and intensity of

effective light treatment.

Although once controversial, it has now been established that light therapy is most effective when

administered in the morning (Eastman et al. 1998; Lewy et al. 1987, 1998). Using the timing of the

onset of melatonin secretion at night, Terman and colleagues have shown that light therapy worksPrint: Chapter 28. Light Therapy

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best when administered between 7.5 and 9.5 hours after the onset of melatonin secretion (J. S.

Terman et al. 2001). A simple questionnaire designed to help patients estimate their own time of

optimal light therapy, is available in the form of a “Morningness–Eveningness” assessment (see

http://www.cet.org). In practice, this means encouraging patients to undertake their light therapy

as early as possible. If they are not able to do light therapy in the morning or if treatment appears

unhelpful when administered at that hour, it is reasonable to encourage light therapy usage later in

the day. There is evidence that light therapy administered in the evening is superior to placebo (M.

Terman et al. 1989b). In addition, several studies of light therapy have found benefit to

administering it in both morning and evening (Rosenthal et al. 1984, 1985). Given the time

pressures that many patients experience in the morning, dividing treatment between different

times of the day might prove most practical.

As noted above, white light appears to be effective with or without the ultraviolet component.

Several studies have investigated the efficacy of light of different colors for patients with SAD.

Thus, white light has been shown to be superior to green light (Stewart et al. 1991) or to red light

or blue light (Brainard et al. 1990), and green light to be superior to red light (Oren et al. 1991).

There has been a recent upsurge in interest in blue light, which may be the most active part of the

spectrum for some physiological responses in humans, such as melatonin suppression (Brainard et

1. 2001). This enthusiasm has led to the commercial development of light fixtures with blue

light–emitting diodes. Claims that such light fixtures have equivalent efficacy to standard

white-light fixtures are unfounded at this time. To date, there are no published studies indicating

that such blue-light fixtures are effective or safe as antidepressant treatments. For these reasons,

the use of blue-light fixtures cannot be recommended at this time.

The light boxes used in research studies have been of varying dimensions, but in most cases the

surface area has been at least 1 foot by 1½ feet. Currently, many smaller versions are being

marketed as “10,000-lux fixtures.” While they may indeed emit light of this brightness when the

eyes are in a certain position, we cannot assume that they will be as effective as their larger

counterparts. Given the popularity of these smaller fixtures, which are cheaper and handier, studies

comparing treatment with boxes of different sizes are certainly warranted. Anecdotally, we have

found that sometimes people who fail to respond to smaller light boxes will respond if switched to

larger ones.

NOVEL TREATMENT DEVICES

One novel way of administering light therapy is by means of a head-mounted light visor. In three

multicenter studies of a light visor (Light Visor, Bio-Brite, Inc., Bethesda, Maryland) involving more

than 200 patients, no difference in efficacy was observed at widely differing intensities (Joffe et al.

1993; Rosenthal et al. 1993; Teicher et al. 1995). Response rates in these studies were comparable

to those in some light box studies, but the absence of any differences between treatment conditions

raises a problem in interpreting these findings. Although controlled studies are lacking, for those

who need portability, a trial of a visor might be warranted.

Another innovative way of administering light therapy is by means of electronic devices that can be

connected to light sources in the bedroom in such a way as to simulate a summer dawn (M. Terman

et al. 1989a). In other words, the electronic device can cause the light source to turn on gradually

at a predetermined time and increase to its maximum intensity over a preprogrammed period. In

their meta-analysis, Golden et al. (2005) considered five dawn-simulation studies, all by Avery et

1. (1992, 1993, 1994, 1998, 2001), and concluded that dawn simulation is an effective treatment

for SAD. These findings are paradoxical given that the patient is asleep while receiving this

treatment and the final light brightness incident on the eyes (250–275 lux) is far lower than that

reported to be necessary for a therapeutic response to regular light therapy. Presumably, the

well-documented and universally experienced increased sensitivity of the eyes to this especially

early morning light accounts for the efficacy of the lower intensities of light delivered by this type

of treatment (Knoerchen and Hildebrandt 1976). Dawn simulation can both improve the patient’s

mood and help the patient wake up more easily in the morning. The currently available dawnPrint: Chapter 28. Light Therapy

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simulator is likely best used as a supplement to regular light therapy, as its efficacy alone is not as

high as that of light boxes alone (Lingjærde et al. 1998). Two studies suggest that negative-ion

generators may also have antidepressant effects in SAD (M. Terman and Terman 1995; M. Terman

et al. 1998).

PRACTICAL ASPECTS OF LIGHT THERAPY

In general, it is best to begin light therapy with the patient receiving 10,000 lux. Lower intensities

are effective but take significantly more time out of each day. It is usually preferable for patients to

obtain commercially available fixtures. Attempts by patients to construct their own fixtures may

result in fixtures that put out the wrong amount of light or are electrically unsafe. Commercially

available fixtures have additional advantages, such as special ballasts that minimize flicker and

features that minimize the transmission of UV rays.

Duration of treatment, like dosage of an antidepressant medication, may need to be adjusted

according to the needs of the individual patient, the time of year, and the amount of ambient light.

These factors are best considered collaboratively with the patient, who gradually becomes more

educated as to her or his need for light. Thirty minutes in the morning is frequently a good duration

to start with—enough to induce some antidepressant response but not so much as to cause side

effects. Although some patients may experience an immediate beneficial effect of light, either an

activation or a lessening of anxiety, most patients take 2–4 days to register a sustained

antidepressant response (Rosenthal et al. 1984). The response to light may be manifested over

several weeks (Bauer et al. 1994; Eastman et al. 1998), and a lack of response within the first week

cannot be taken as a certain indication that a patient will not derive any benefit from light over the

long run. Light treatments can be administered in divided doses, and some patients report this to be

more effective and convenient than when treatment is consolidated in a single block at one time of

day. Shifting light treatment earlier may improve the likelihood of response.

Several researchers, seeking predictors of response to light therapy, have found a history of

hypersomnia, a preponderance of atypical vegetative symptoms, and increased intake of sweet

foods in the afternoon to be predictors of a favorable response to light treatment (Kraüchi et al.

1993; Oren et al. 1992; M. Terman et al. 1996). As much as 40% of the variance in the

antidepressant response to light therapy may be accounted for by hypersomnia alone. Another

clinical predictor of a favorable response is a history of reactivity to ambient light. For example,

observations of mood improvement following trips south in the winter or varying depths and

durations of winter depressions experienced when living at different latitudes would suggest a

favorable response to light therapy.

Side effects of light therapy can be reduced by decreasing exposure to light, either by decreasing

duration or by suggesting that the patient sit farther away from the light source. Insomnia may be

most pronounced when light therapy is administered at night, and it may be obviated by moving

light treatment to an earlier time of day. Damage to the eyes has been discussed as a potential side

effect, as noted above, although it has not been reported to date after properly supervised light

therapy, even among patients who have been treated for several years (Schwartz et al. 1996). A

good history of any eye-related problems should be taken before starting a patient on light therapy

(M. Terman et al. 1990). At this time there are no data to suggest specific guidelines for how

frequently the eyes should be examined in patients being treated with light therapy.

Clinical experience suggests that light therapy can be combined with antidepressant medications to

good effect. Although such combinations have not yet been formally tested, this strategy often

permits the use of lower doses of medications, which results in fewer side effects.

CONCLUSION

Light therapy is widely regarded as useful for patients with SAD. Furthermore, there is evidence

that it may prove to be a versatile form of treatment, valuable also in those with other types of

mood disorders and sleep and eating disorders. Light therapy may be used either alone or in

conjunction with medications. It can be titrated like a medication in regard to both dosagePrint: Chapter 28. Light Therapy

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(duration and intensity) and timing of administration. Our understanding of light therapy and of the

techniques and technologies involved in administering such therapy has advanced significantly over

the past 25 years. There is every reason to expect that such progress will continue.

Many resources exist to keep interested clinicians and researchers up-to-date. These include a

professional society, the Society for Light Treatment and Biological Rhythms

(http://www.sltbr.org), and the Web site of one of the authors

(http://www.normanrosenthal.com). For further information, the reader is referred elsewhere

(Rosenthal 2005; Zulman and Oren 1999).

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