Lack of sleep leads to lost kilojoules

Skipping a night of sleep is equivalent of the amount of energy it would take to walk about two miles, new research suggests.

Alternatively, looked at in terms of food, a night of sleep deprivation can be translated into losing about 567kj - about the amount found in two slices of bread.

"While the amount of energy saved during sleep may seem small, it was actually more than we expected," said study author Professor Kenneth Wright, director of Colorado University's Sleep and Chronobiology Laboratory.

"[And] if one considers the amount of positive energy storage needed to explain the obesity epidemic is 210kj a day, the energy savings represented by sleep is physiologically meaningful," he added. Wright and his colleagues report their findings in The Journal of Physiology.

Sleep and weight gain

The team's results are based on work with seven young adults who were tracked while they spent three days in bed while placed on a weight maintenance diet.

While the participants slept a full eight hours on day one, on days two and three they were deprived of sleep for a total of 40 hours, after which they recovered with eight hours of sleep.

The authors found that over the course of 24 hours of sleep deprivation, the participants expended 7% more energy than they would during a normal night of sleep.

Wright and his associates said the finding suggests that the normal sleep-wake cycle is linked to a typical use of body energy, and that depriving the body of sleep appears to siphon off some of that energy.

On the other hand, the team said, the finding raised questions about why people don't save even more energy while asleep.

"There are other functions of sleep that are important and cost energy," Wright pointed out. "Some conserved energy may be re-distributed to support vital physiological processes, like learning and memory consolidation, immune function and hormone synthesis and release."

And, he cautioned, sleep deprivation should not be thought of as a means to lose weight, since prior research shows that a lack of sleep is linked to both cognitive impairment and weight gain.

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