The effects of light on the circadian rhythms depend on the following:

Intensity. There is probably an exponential or logarithmic relationship between the intensity of the light exposure and its effect in modifying circadian rhythms. Exposure to 25lux for around 20min will entrain the circadian rhythm in around 75% of subjects, and while the maximum effect can be seen with around 1000lux, around 50% of this is obtained with only 100lux, which is less than the light level in most domestic situations during the day (

Table 2.3).

Duration. Brief light pulses can influence the circadian rhythms, but continuous exposure for 10–20min has a significantly greater impact.

Wavelength. The individual wavelengths of light influence the circadian rhythms to different extents.

The most effective in resetting them are between 445 and 475nm, particularly around 460nm, which is close to the wavelength to which melanopsin in the retinal ganglion cells is most sensitive.

Timing. The sleep phase can be shifted by up to 1–2 hours per day by light exposure according to its timing. The phase response curve of sleep to light is complex (

Fig. 2.5). The circadian rhythms are insens-
itive to light exposure during the day. There is little sleep phase delaying effect before around 9.00 pm or phase advance after around 10.00 am. Light exposure at night, when there is normally little light and when melatonin is secreted, can have a major effect on the circadian rhythms. A rapid shift in the response to light occurs around the nadir of temperature and at the time of peak melatonin secretion, usually 3.00–5.00 am.

Light exposure before this phase-shift transition point delays the next sleep phase, but afterwards leads to a sleep phase advance. The magnitude of both of these effects is greatest close to the temperature nadir.