Humans differ widely in their sensitivity to low levels of light in the evening, which could explain why late exposure to artificial light worsens the sleep and health of some — but not all — people.
Sean Cain and his collaborators at Monash University in Melbourne, Australia, exposed 55 people to varying levels of light starting from four hours before their bedtimes, and periodically measured the amount of the hormone melatonin in the participants’ saliva. Melatonin levels naturally rise in the evening, helping to start the sleep cycle, but are suppressed by light.
Exposure to indoor light levels lowered the volunteers’ melatonin levels by half, on average, delaying the hormone’s rise until later in the evening. The brighter the light, the longer the melatonin rise was delayed. But the volunteers’ sensitivity varied greatly — even dim light one-fifth the brightness of normal indoor light could delay the melatonin surge in some individuals.
The researchers say that the findings could help to explain why some people are prone to circadian-rhythm disruption, which has been linked to illnesses such as bipolar disorder.