Long-term lack of sleep can make you feel a lot worse than a bit bad tempered and under the weather. It's something we just can't do without: deprived of sleep's restorative benefits, rats die prematurely. According to Richard Stephenson of the University of Toronto, `Sleep presents one of the great mysteries that remain in biology'; researchers still aren't exactly sure what the function of sleep is. The metabolic rate in sleep deprived rats nearly doubles and they produce a lot of excess heat, so it's possible that lack of sleep affects the rats' temperature regulation. But other mechanisms could be at work, so to see if the rise in metabolic rate is to do with temperature regulation or not, Stephenson and his colleagues Karen Chu and James Lee investigated sleep deprivation in an animal that can't regulate its body temperature, the Pacific beetle cockroach (p. 2540).

Insects don't sleep in the same way that mammals do, but they do undergo periods of `sleep-like rest' where they sit very still and don't interact with their environment. The team found in preliminary tests that a combination of a small movement and a puff of CO₂ alerted dozy cockroaches and kept them awake, probably because it exploits natural predator avoidance behaviour and the cockroaches never stop responding to the stimulus. They also found that sleep deprived cockroaches only needed 55 s to nod off again after being disturbed from a 2 min snooze. Normal roaches took 356 s, showing that sleep deprivation increases need for sleep.

To test how the cockroaches coped with sleep deprivation, the team placed them in cylinders equipped with food and water. The sleep deprived cockroaches received a CO₂ puff and a 2 s, 1 cm rotation of the cylinder every minute to keep them awake. The other group received the same number of puffs and rotations in the day, every 30 s for 3 hours in each 6-hour period, giving them four 3-hour rest periods each day.

To measure how the sleep deprivation was affecting survival, the team counted the number of dead cockroaches each day. The normal roaches died at a rate of 1 every 7.7 days for the whole experiment. The sleep deprived roaches all survived up to day 17, but after that started dying at 1 every 1.57 days, showing that sleep deprivation increased the risk of dying young.

To find out how sleep deprivation affected metabolic rate, the team removed the cockroaches from their containers at the end of each week and put them individually into a specially designed respirometer to measure oxygen consumption, and hence their metabolic rate. While metabolic rates were the same at the beginning of the experiment, in the sleep deprived group metabolic rates were 82% higher after 35 days. This shows that the metabolic rate rockets in sleep deprived insects, suggesting that in cockroaches at least, there is a change in metabolism which results from the sleep deprivation, and which isn't related to temperature regulation. To try and get to the bottom of the mystery, researchers `will need to find the source of the heat', Stephenson says.