London: Body clock function can break down when light and temperature levels
throughout the day are out of sync, finds new UCL research in fruit flies. The study, published in Cell
Reports, examined the activity levels of flies when they were exposed to
light and heat at different times. The ‘natural’ cycle involved 12 hours of
light and warmth followed by 12 hour of darkness and cold. The light and
temperature cycles were then varied so that the light cycle ran between two and
10 hours behind the temperature cycle in each experiment.
When the time lag was small the effect on the body clock was relatively
low, following an average of the light and temperature readings. When the time
lag was large, temperature was ignored and the body clock was set by light. A
moderate time lag of around six hours, however, caused the clock function to
break down, with major disruptions to behaviour and clock-controlled molecular
signals.
“The body clock is a naturally evolved entity so it functions best within
naturally-occurring conditions,” explains lead author Ross Harper (UCL
CoMPLEX). “Changes in sunlight cause changes in temperature, and both are used
to sense time. The time lag between light and temperature naturally varies, so
the clock can compute an average time for smaller changes. For very large
differences, such as a heat spike at midnight, the system ignores temperature
as it is no longer a reliable indicator of ‘daytime’. In between these lies a
zone of confusion, where the clock can’t combine the cues anymore but can’t yet
ignore them either. As mammals, including humans, are subject to similar
evolutionary constraints, it is likely that similar systems apply to our
clocks.”
In the natural cycle, flies became increasingly active throughout the 12
hours of daylight with a peak in the evening just before light and temperature
levels dropped. However, when there was a six-hour time lag between temperature
and light, the flies were only active in the six-hour window when it was both
cold and light. There were no peaks in activity, simply a plateau of moderate
activity.
“Light is a well-known driver of the body clock, but until recently other
factors including temperature have been overlooked,” says co-senior author
Professor Joerg T Albert (UCL Ear institute). “Recent work has shown that the
daily body temperature rhythm is important for setting the human body clock,
and our study suggests that the difference between light and temperature may
also have a role to play. Artificial exposure to light sources and irregular
sleep/wake patterns such as shift work shoulder a lot of the blame for
disrupting the body clock, but artificial temperature controls like air
conditioning and central heating perhaps also have an influence.”
One complication with studying temperature in humans and other mammals is
that our bodies regulate temperature internally, whereas an insect’s body temperature
depends solely on the outside temperature. It is not currently clear how
internal and external temperatures relate to each other and the body clock, so
further research is needed to better define their relative contributions in
humans.
“Modern life involves artificial control of many aspects of our
environment, but more natural conditions are likely to benefit our body
clocks,” explains co-senior author Professor Ralf Stanewsky (UCL Cell &
Developmental Biology). “There are simple ways to mimic more natural
conditions, such as having colder bedroom temperatures and getting as much
light in the morning as possible. People with difficulties should avoid coffee
and energy drinks where possible, drink less alcohol, avoid eating or
exercising late at night and try to keep regular bedtime hours. The aim should
be to have a good night's sleep and naturally wake up 'on time' in the morning
without an alarm clock. The body clock is important, and disruption can cause
not only sleeping problems but also certain psychiatric and cognitive
disorders.”
The research was funded by the European Research Council, the
Biotechnology and Biological Sciences Research Council and Engineering and the
Physical Sciences Research Council.