Light exposure
Light exposure has complex and important effects on sleep and wakefulness _{[14, 15]}. The seasonal changes in the duration of daily light exposure lead to cyclical alterations in the duration of rest and activity. The duration of melatonin secretion and of sleep is longer in winter than in summer in non-tropical latitudes.

At high latitudes with prolonged nights the long nocturnal sleep phase in winter may break into two episodes with an intervening stage of quiet wakefulness. Seasonal changes in light duration also influence mood and behaviour, as exemplified by the seasonal affective disorder.

During sleep, 5–10% of ambient light reaches the retina even through closed eyelids. This has an alerting effect during NREM sleep, increasing the electroencephalogram frequency and causing arousals from sleep, with the result that there may be a later rebound increase in NREM sleep. The effects on REM sleep are less clear, but this can be promoted by brief episodes of darkness.

During wakefulness, light exposure has several important physiological effects that are independent of vision and visual pathways to the brain. These include an increase in the level of alertness, which is particularly marked with bright light and at night.

Light exposure also significantly improves motor performance, for instance in shift workers, and elevates mood. Acute light exposure also increases the heart rate and core body temperature.

A separate non-visual effect is the influence of light on the timing of the circadian rhythms. Exposure to light resets these each day so that they are coordinated with environmental time. These effects are mediated by the influence of light on the suprachiasmatic nuclei and its suppression of melatonin secretion (

Fig. 2.4).