An hour of bright white light in the early morning improves performance and advances sleep and circadian phase during the Antarctic winter
Previous work has demonstrated that exposure to an hour of bright light in the morning and the evening
during the Polar winter has beneficial effects on circadian phase. This study investigated the effect of
a single hour of bright white morning light on circadian phase, sleep, alertness and cognitive performance.
Nine individuals (eight male, one female, median age 30 years), wintering at Halley Research
Station (75◦S), Antarctica from 7th May until 6th August 2007, were exposed to bright white light for a
fortnight from 08:30 to 09:30 h, with two fortnight control periods on either side. This sequence was performed
twice, before and following Midwinter. Light exposure, sleep and alertness were assessed daily
by actigraphy, sleep diaries and subjective visual analogue scales. Circadian phase (assessed by urinary
6-sulphatoxymelatonin rhythm) and cognitive performance were evaluated at the end of each fortnight.
During light exposure circadian phase was advanced from 4.97
±
0.96 decimal hours (dh) (mean
±
SD)
to 4.08
±
0.68 dh (p = 0.003). Wake-up time was shifted by a similar margin from 8.45
±
1.83 dh to
7.59
±
0.78 dh (p < 0.001). Sleep start time was also advanced (p = 0.047) but by a lesser amount, consequently,
actual sleep time was slightly reduced. There was no change in objective or subjective measures
of sleep quality or subjective measures of alertness. An improvement in cognitive performance was found
with both the Single Letter Cancellation Test (p < 0.001) and the Digit Symbol Substitution Test (p = 0.026)
with preserved circadian variation. These beneficial effects of a single short duration light treatment may
have implications not only for the Antarctic but other remote environments where access to natural light
and delayed circadian phase, is problematic. These results require validation in larger studies at varying
locations.