At first thought, it's hard to see why spending less time in bed than more should make us flabby or affect our growth.
But a steady diet of research demonstrates that sleep deprivation and duration influence human physiology from head to toe.
Swedish researchers recently reported that having students miss a full night's sleep made their metabolism slow by 5 percent to 20 percent the following day, based on measurements of appetite, blood sugar and hormone levels and compared with the day after a normal night's sleep. The research appeared in the American Journal of Clinical Nutrition.
Between 50 million and 70 million Americans suffer from chronic sleep loss and sleep disorders, according to the National Institutes of Health.
Another study out in May's issue of the journal Sleep confirms what parents and grandparents have long claimed: Babies can grow like weeds overnight, especially if they snooze a long time.
The study involved 23 parents who recorded daily sleep patterns for their healthy infants starting at about 12 days old and continuing for four to 17 months. The babies' length and weight was also regularly recorded.
The results clearly showed that infants slept longer -- about 4 1/2 hours -- and more often - three extra naps a day for two days -- as they experienced significant growth spurts. In all, they found that the probability of a growth spurt increased by 43 percent for each additional sleep episode and 20 percent for each additional hour of sleep.
"This shows that growth spurts not only occur during sleep, but are significantly influenced by sleep," said Michelle Lampl, a anthropology professor at Emory University and lead author of the study. "Longer sleep corresponds with greater growth in body length."
The researchers noted that some sleep patterns changed in babies who did not have a growth spurt and that not every growth spurt was preceded by a burst of sleep. The scientists also reported in another study this month that they had recorded growth spurts in the head circumference of infants associated with sleep changes.
Still another study, done in rats, found that often, when the lights go out in a tired brain, the outage only affects certain neighborhoods.
Researchers at the University of Wisconsin-Madison documented that some nerve cells in a sleep-deprived, yet alert, brain can briefly go 'off line," into a sleeplike state, while the rest of the brain appears to still be wide-awake.
Scientists had thought that sleep deprivation generally hits the entire brain, with monitors showing patterns typical of either being awake or asleep. The results were published in the April 28 issue of the journal Nature.
"Even before you feel fatigued, there are signs in the brain that you should stop certain activities that may require alertness," said Dr. Chiara Cirelli, a professor of psychiatry who led the study. "Specific groups of neurons may be falling asleep, with negative consequences on performance."
Cirelli said scientists know that when people are sleepy, they make mistakes and lose vigilance. "We have seen with electroencephalograms that even while we are awake, we can experience short periods of 'micro sleep."' But the new research, she said, shows that even before that stage, brains may already showing signs of sleep-like activity that impairs them.
The selective shutdowns may also explain the lapses in judgment, attention span and general irritability that many people feel when they've been deprived of sleep, even though they may not feel particularly sleepy.
When the researchers tested motor skills of the sleep-deprived rats, "we saw the rats start to make mistakes," Cirelli said, such as dropping or missing their reach for food pellets.
Monitoring brain cells during the tests, the scientists saw periodic shutdown of just a few cells apparently leading to mistakes. For instance, in one test, 18 out of 20 neurons stayed awake. In the other two, brief periods of activity alternated with periods of silence.
(Contact Lee Bowman at BowmanL@shns.com.)
(Distributed by Scripps Howard News Service, www.scrippsnews.com)