The human wake-sleep cycle is governed by circadian rhythms. Your internal clock (an area in your brain that scientists call the suprachiasmatic nucleus or SCN) tells your body when it’s time to sleep and when it’s time to be awake. But how does it know?
Our circadian system is usually synchronized with the solar day ensuring that wakefulness peaks during daylight hours and a long period of sleep occurs during the night. Light triggers changes in body temperature, alertness, and the release of hormones. However, for people with circadian rhythm sleep disorders, the pattern can become misaligned causing insomnia, fatigue, and poor performance at work or school.
Bright light therapy (BLT) can help reset the internal clock in those with sleep disorders. When you are exposed to bright light at appropriate times, it can improve sleep quality and daytime alertness. In this article, I provide an overview of the research done linking BLT with improvement of circadian rhythm sleep disorders.
Light treatment is delivered to the brain through the eyes. When certain ocular photoreceptors are exposed to natural light or artificial bright light, they are activated and cells in the retina deliver the light directly to the circadian clock in the SCN.
The efficacy of BLT for treating sleep disorders depends on the dose of the light (meaning, the intensity and duration of the light), as well as the time of day the light is administered. Intensity of light is measured in “lux”: a bright sunny day could reach 10,000-30,000+ lux, an overcast day could measure around a few thousand lux, and time spent indoors is generally between 100 and 1000 lux. In 1998, a study was done to measure the minimum light intensity to suppress nocturnal melatonin concentration in human saliva: the study found that 393 lux at 30 minutes, 366 lux at 60 minutes, 339 lux at 90 minutes, and 285 lux for 120 minutes were each enough to suppress melatonin release in humans. Another study in 2019 found that 50% suppression of melatonin occurred at <30 lux, which is comparable to or lower than indoor lighting used at night, as well a light produced by electronic devices. This means that inadvertent use of low intensity light close to bedtime may be enough to suppress melatonin release, and therefore delay sleep onset. However, in most studies, when the use of light was planned with the intention of improving mood or sleep quality, only exposure to bright artificial light (typically 2,500 to 10,000 lux) was effective for these goals.
It’s important to recognize that BLT works best with those experiencing sleep disorders caused by circadian rhythm misalignment. This means that it is recommended for patients whose sleep-wake cycle (bedtime and wake time) is inappropriately timed relative to the biological night. The circadian system is most sensitive to light during the biological night, during a time when most people are sleeping in darkness. Thus, light therapy is most effective in treating circadian rhythm misalignment if administered shortly after awakening, or shortly before bedtime, depending on the type of sleep disorder. A list of these types of sleep disorders and short descriptions follow.
Delayed sleep-phase (DSPD) disorder is a condition which shifts the normal sleeping pattern outside what is considered the social norm. People who have DSPD have delayed sleep onset and wake times in relation to their desired sleep schedule; they generally can’t fall asleep until several hours after midnight (2:00-6:00 a.m.) and then have trouble waking up in the morning, making adhering to a normal work or school schedule difficult. The most common onset of this condition is in childhood or adolescence. The diagnostic method used is a sleep log (noting down time to bed, time to wake, duration of sleep, all naps) for a minimum of seven consecutive days.
Once diagnosed, the desired outcome here is falling asleep earlier and waking up earlier. Two methods of accomplishing that include bright light therapy given shortly after awakening, and melatonin given before the desired bedtime. Researchers found that 2,500 lux for 2 hours or 10,000 lux for 30 minutes in the morning were enough to achieve an early sleep onset for people with DSPD. Additional treatments include ingesting melatonin 1 to 2 hours before the desired bedtime, as well as avoidance of any light sources (since even dim indoor lighting at <30 lux can suppress 50% of melatonin release) for at least 1 to 2 hours before the desired bedtime.
In advanced sleep-phase disorder (ASPD), the sleep episode occurs much earlier than the patient’s desired sleep-wake times. Patients with ASPD experience difficulty remaining awake in the evening and often complain of early morning awakenings during sleep. Typically, individuals presenting with ASP go to sleep between 6:00-8:00 p.m., and wake between 1:00-3:00 a.m., despite attempts to delay sleep-wake times to later hours. This condition is rare, and typically occurs in the elderly. The diagnostic method is the same: a sleep log for a minimum of seven consecutive days.
Here, in order to delay sleep, experts recommend patients with ASPD be exposed to bright light therapy shortly before bedtime. This can delay sleep time by two hours and help patients avoid waking too early.
Individuals with free-running disorder (FRD) typically have a longer than 24-hour circadian rhythm. This condition is rare, and is most common in blind people. Since light cues are either less available or unavailable to them, sleep disturbances in the blind are common. In fact, the degree of visual loss is related to the occurrence of free-running disorder. The insomnia and daytime sleepiness that occur when the circadian rhythm is misaligned with the desired sleep time have been noted as being second in debilitation to the blindness itself.
FRD is thought to be rare among those with normal eyesight. However, a good proportion of blind individuals maintain some light perception, and/or are able to align their circadian rhythm by using their social and activity schedules as a replacement for “light cues”.
The treatment options for this condition are similar to those with ASPD and DSPD. For sighted patients, the use of planned sleep schedules, timed bright light exposure soon after waking up, and melatonin administration in the biological evening are core treatment options. For those who are blind, the primary therapies are timed melatonin administration in the biological evening, plus the use of social and activity schedules (instead of light) as the main “cues” to train the body on when to sleep.
For those who work the night shift, it can be difficult to get consistent, restful sleep. Those with shift work disorder often complain of insomnia when they attempt to sleep during the daytime, and excessive sleepiness when they attempt to work during the nighttime. These symptoms result from the inability to adapt the timing of their circadian rhythm to their imposed work schedule: sleep propensity peaks during the work shift, resulting in fatigue, increased risk of work-related accidents, and a higher rate of single-vehicle car crashes on the drive home after work. In addition, the circadian rhythm of alertness peaks during the daytime, resulting in insomnia during daytime sleep and less total hours of sleep.
Loss of sleep and circadian rhythm misalignment contribute to an increased risk of developing gastrointestinal disorders, metabolic syndrome, diabetes, heart disease, and cancer among shift work populations. However, circadian rhythms can adapt to working the night shift and these risks can be minimized.
Adaptation requires strict adherence to regular sleep and wake schedules (even on days off), exposure to bright light during work shifts, avoidance of outdoor morning light at the end of the shift (via sunglasses or tinted goggles), and darkness during scheduled sleep times. In simulated shift work studies, in which research subjects were exposed to bright light during the work shift (approximately 10,000 lux for 7.5 hours, 4 consecutive days) and darkness during sleep at home, circadian rhythms of body temperature, cortisol, and subjective alertness realigned with the imposed night shift schedule, whereas exposure to room light (instead of the bright light at 10,000 lux) for those 7.5 hours did not result in these same adaptations to the shift work schedule.
In two other studies, night shift nurses were exposed to bright artificial light in the evening (approximately 5,000 lux for 4 hours or approximately 3,000 lux for 6 hours) and to reduced outdoor morning light after the end of the work shift (via sunglasses or tinted goggles). After daily administration of this regimen, these nurses reported increased alertness at night during their work shift, increased amounts of daytime sleep, and the circadian rhythms of melatonin and body temperature shifted by greater than 9 hours, demonstrating a biological adaptation to the night shift schedule. In contrast, exposure to normal room light (and not bright light) during the work shift and morning bright light after work did not result in adaptation to night shift work.
These studies show that shift work disorder can be treated by appropriately timed exposure to light and darkness and regularly scheduled sleep.
Jet lag disorder happens after swift airline travel over multiple time zones, and is due to a temporary misalignment between one’s own internal circadian clock relative to the pattern of light and darkness in the new time zone. Although jet lag is typically considered to be a simple inconvenience for most travelers, it can be a chronic and debilitating condition for pilots, flight crew, military personnel, and frequent business travelers.
The severity of jet lag depends on the number of time zones crossed, the direction of the flight (eastward vs. westward), and the quality and amount of sleep obtained in-transit. When traveling east, most individuals have trouble falling asleep at the desired time since the new time zone is several hours later/ahead than their internal sleep clock. As a result, a “phase advance” is required, which means exposure to BLT should occur shortly after awakening, and dim light should be experienced before the desired bedtime. When traveling west, the problem is the opposite: one internal clock is set for a bedtime and wake time that is far earlier than the new time zone, and as a result, early morning awakenings are common during the initial time zone adjustment. To address this, BLT should be timed in the evening hours, before the time of one’s normal sleep onset in their home time zone, and darkness/dim light should be favored for the hours after awakening.
Currently, there are no standard guidelines governing the application of light therapy for the treatment of circadian rhythm sleep disorders. There are, however, basic safety standards and recommendations borrowed from the application of BLT in the treatment of Seasonal Affective Disorder (SAD).
Studies show that light therapy should be sufficiently bright (2,000-10,000 lux) to be beneficial, but also short enough in duration (less than 2 hours) to ensure patient compliance. Some more recent studies have shown that blue light is more effective at resetting circadian rhythms than white light. But more research is needed on light wavelengths.
Due to the increase in popularity of BLT for the treatment of seasonal depression, a wide variety of light therapy devices have entered the market including white light boxes, light visors, dawn simulators, and colored light boxes (e.g., blue LED boxes). Unfortunately, many of these devices have not been clinically tested for their efficacy or safety. It is therefore important to select a light therapy device that has tested well in one or more peer-reviewed clinical research studies.
Importantly, light therapy devices must provide for adequate filtering of UV and infrared light, so as to avoid ocular damage, and the spectral emission of the light source should be provided by the manufacturer. The light source must be bright enough such that the treatment is effective (e.g. 10,000 lux at approximately one foot from the device), and also large enough such that the eyes remain in the therapeutic range of the light even with small movements of the head.
For reference, the Center for Environmental Therapeutics website (www.cet.org) provides specific guidelines for selection of light therapy devices, and an “Ask the Doctor” forum for clinician and patient questions regarding the proper application of light therapy.
Bright light therapy is a simple, yet effective, treatment for circadian rhythm sleep disorders. However, it does require discipline and persistence to follow strict, time sensitive guidelines on timing of BLT, melatonin ingestion, sleep time and wake time on a daily basis. However, if you’re having trouble going to sleep or waking up at the desired time, and you want to try a treatment that is effective and does not require prescription medicines, BLT might be worth a try.