SLEEP: Sleeping Late on Weekends Induces Monday Fatigue

SALT LAKE CITY, June 22 - Don't change your alarm clock for the weekend, recommends an Australian researcher. Sleeping late on Saturday and Sunday mornings may make you more tired and cranky on Monday and Tuesday.

But before he was booed off the stage at Sleep 2006, the joint meeting of the Sleep Research Society and the American Academy of Sleep Medicine, Leon Lack, Ph.D., of Flinders University in Adelaide backtracked a bit.

He protested that he was not attacking the traditional weekend snooze. "I just want people to realize what the consequences might be if they sleep in very late on the weekend," he said.

Those consequences, he reported, are a significantly greater level of fatigue and sleepiness on the following Monday and Tuesday. That, in turn, helps contribute to a sleep debt that isn't paid off until the weekend, he said-when people try to get back on an even keel by sleeping late.

"The vicious cycle starts repeats," he noted.

The finding makes sense, commented sleep researcher Helen Burgess, Ph.D., of Rush University Medical Center in Chicago, because it's known that altering bedtimes and wake-up times can change a person's body clock. Dr. Burgess and colleagues reported earlier this year that delaying the wake-up times of 14 healthy participants over a two-week period shifted their body clocks by 144 minutes.

"Wake time has a much bigger effects on circadian rhythm than bedtime," Dr. Burgess noted. "If you stay up three hours later, you get a half-hour delay in the (circadian) phase, but if you wake up three hours later, you can get a two and a half hour delay."

Dr. Lack and colleagues enrolled 16 young adults and studied their circadian rhythms when they arose at their normal weekday time on the weekends (the control condition) and when they were allowed to sleep as late as they wanted. The researchers used a well-known marker called the dim light melatonin onset (DLMO), which is used to assess the changes in the circadian rhythm.

The volunteers were also asked to rate themselves during the week on the Daytime Sleepiness Scale (DSS) and a fatigue scale.

The researchers found that sleeping late on the weekend-the average extra time was about three hours-had delayed circadian rhythms by 32 minutes on average by the time they went to work on Monday. The difference between that delay and the circadian rhythm in the control condition was significant at P=0.001.

The effect of the delay, Dr. Lack said, was that participants were sleepier on Monday and Tuesday if they had slept late on the weekend; on average they scored a four on the DSS on Mondays, compared with less than three when they hadn't slept late.

Also, they felt more fatigued. On the Monday after sleeping late, they scored two on the fatigue scale, compared with about 1.5 when they had not slept late.

Both effects were also seen on the Tuesday, but wore off as the week went on.

Most sleep researchers, Dr. Burgess commented, tend to think that a change in the circadian rhythm of less than an hour has little effect. "What I found most interesting about the research," she said, "is that he showed that that half-hour difference in circadian rhythm can be felt by people."

Both Dr. Burgess and Dr. Lack suggested an alternative to sleeping late: Arise at the normal weekday hour and-if you feel sleepy later-have a nap.

A midday snooze won't reset your body clock and will help you repay your sleep debt, Dr. Lack said. And Monday morning won't be so painful.