Light therapy phototherapy luminotherapy

Exposure to bright light has an alerting effect and leads to sleep phase changes. These are influenced by the following.

Timing of light therapy

The effect of light on the sleep phase is determined by the phase response curve (page 31). This has an inflection point at around the time of the temperature nadir (3.00-5.00 am). Light has more effect on changing the phase of sleep when it is applied close to the inflection point. Before this time it delays the onset of sleep and afterwards it causes a sleep phase advance. Light exposure during the middle of the day has virtually no effect on the sleep phase, but it does increase alertness.

The 'time of light exposure' is conventionally taken as the mid-point of the exposure to light. This is particularly important if it is applied close to the inflection point since the onset of light exposure may, for instance, be before the inflection point, despite the mid-point falling afterwards.

Light therapy is most effective in advancing the sleep phase if it is given between 6.00 and 10.00 am. As the wake-up time becomes earlier so the timing of the light therapy can be moved earlier to further advance the phase shift. Once the circadian phase has been modified, light should be presented at a consistent time each day to maintain this change.

Light intensity

The sleep phase shift produced by light therapy is approximately proportional to the log of the light intensity. The maximum response is seen with around 1000 lux and 50% of this with around 100 lux, but even 25 lux can alter the sleep phase. Dawn stimulation, with a gradually increasing light intensity, may be particularly effective, even if the eyes are closed, because 5-10% of light reaches the retina through the eyelids. An intensity of 2500 lux is usually given for 2 h, or 5000 lux for 1 h or 10 000 lux for 30 min, but high intensities of light exposure can cause eye damage.

Duration of light treatment

The phase shift is approximately proportional to the duration of light exposure up to around 3 h, but brief exposures for even around 1 min can have a significant effect.

Wavelength

The retinal ganglion cells which project to the sup-rachiasmatic nuclei have a peak sensitivity to light of wavelengths 445-475 nm, especially around 460 nm. This bluish light is most effective in shifting circadian rhythms and suppressing melatonin secretion. Ultraviolet light is not required and should be filtered out since it can cause cataracts and skin cancer.

Bright lights in the bathroom, bedroom and kitchen, and encouragement to walk to work may help, but sitting by a window and exposure to ordinary indoor electric light are insufficient. Light therapy is best delivered from a fluorescent light which is situated on a white or light-coloured surface to increase reflection, or from a visor or mask which can be worn on the head. Fluorescent bulbs are more energy-efficient and produce less heat, but are noisy and flicker. They have diffusers which enable the light to reach a wide area of the retina, in contrast to incandescent light bulbs which tend to give a point source, which stimulates a smaller area of the retina and has less effect. It is usual for the light source to be 1 m from the patient and at eye level. The light does not need to be directed along the line of gaze, and its projection onto the nasal side of the retina inferiorly may produce more melatonin suppression than other orientations.

Side-effects include headache and agitation. Hypo-mania may develop in bipolar disorders and there is a risk of eye damage with high intensities of light exposure.

Light therapy should be combined with sleep hygiene, particularly with regular sleep-onset and wake-up times, and can be used in conjunction with chronotherapy. It is particularly effective in seasonal affective disorder. Once the DSPS has been controlled, the maintenance dose of light may be reduced or the frequency of light therapy decreased to two to three times per week.

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