P-168 Early-evening light mitigates the sleep disruptive effects of late-evening light exposure

The widespread use of electric light and electronic devices has
resulted in extended exposures to light during the late-evening
and at night. Such light exposures acutely suppress melatonin
and sleepiness and result in phase delays of the circadian rhythm.
We investigated whether the acute effects of late-evening light
exposure on our physiology and sleepiness are reduced when
this light exposure is preceded by bright light in the early evening.
Hereto twelve healthy young females were included in a randomised
crossover study. All participants underwent three evening (18:30-
00:30) sessions with different lighting conditions: (A) dim light,
(B) dim light with a late-evening (22:30-23:30) light exposure of
750 lx, 4000 K, and (C) the same late-evening light exposure,
but now preceded by early-evening bright light exposure (18.30-
21.00; 1200 lx, 4000 K). During the sessions melatonin, subjective
sleepiness, body temperature and skin blood flow were measured.
Late-evening light exposure reduced melatonin levels and subjective
sleepiness and increased skin temperature gradients as compared
to dim light. Interestingly, these effects were reduced when the lateevening
light was preceded by a 2.5-hour bright light exposure in
the early evening. Our results show that the acute sleep-disruptive
consequences of a late-evening light exposure can be mitigated by
a single bright light exposure during the preceding (early-evening)
hours. Further research is needed to investigate whether a single
early-evening light exposure can also reduce the phase delaying
effects of a light exposure in the later evening/night.