Teenagers who are weekend lay-abeds are not being lazy, contrary to conventional wisdom and a lot of irritated parents. According to Richard P. Allen, co-director of the Johns Hopkins University Sleep Disorders Center, they're just making up for the sleep deprivation caused in part by their school schedules.

Allen, an assistant professor of neurology at Hopkins School of Medicine, says that as a group, adolescents tend to be naturally sleepier in the morning and wide awake late in the day.

In a study of 150 Baltimore area adolescents, Allen and his colleagues found that the teenagers slept an average of 6 1/2 hours a night, which, he said flatly, "is not enough."

Allen said he does not find it at all surprising that the kids have a hard time waking up, especially on Monday mornings, even though they tend to sleep about nine hours on Friday and Saturday nights -- after staying up past midnight.

Allen compared two high schools in the Baltimore area, one with a starting time of 7.30 a.m. and the other an hour later. "It turns out that the earlier-starting schools end up with the kids having a lot more problems with short sleep during the school week.

The kids appear to be unable to adjust their schedules," he noted. Youths at high schools with a later starting time had less of a problem, Allen said.

Older people also often have trouble getting up on Monday mornings, Allen said, but adolescents and young adults seem among the groups most affected. For that reason, Allen said, he is "not certain that 7:30 is a good time to start the school day. It's probably fine for adults and "for older adults it's great -- for example, for principals and administrators -- but it's not so good for the kids," he added.

Allen said that about 80 percent of teenagers say that the sleepiest time of day for them is between 7 and 10 a.m., with 8 a.m. as the peak.

Younger children tend to adjust to earlier morning times with greater ease, said Allen, so the common practice in areas where many children are bused to school -- starting school earlier for older students and later for younger ones -- is exactly backward from the way youngsters' cycles work at various ages.

Allen, the father of five, is especially concerned about the chronic sleep deprivation of the nation's teenagers.

"There is a myth that the real man in our society can handle his whiskey, his women and his sleep loss," said Allen, who dismisses it with a laugh.

But what isn't a myth is that "our young people are running around sleep-deprived during the school year." Adolescents have a high rate of automobile accidents, he said, in part because they are inexperienced drivers who have not learned rote behavior patterns but also because they lack sufficient sleep.

But it is not just a problem for teenagers. Allen noted that the common difficulty people have falling asleep Sunday night and getting up Monday morning is often a biological, not a psychological, phenomenon.

It is called "delayed sleep phase syndrome" and involves a difficulty in "synchronizing sleep patterns with social demands." Normally, he said, "there are a lot of things that the body uses to help it know when it should sleep and when it shouldn't." These include social cues, routine habits, activity and sunlight. But, Allen said, most people "can generally live pretty nicely on just social cues."

However, for a lot of people, the mechanisms that govern the circadian cycles may be slightly out of synch. "In a mild case, people will tend to go to sleep late on weekends and then try to go to bed early the night before work. They probably won't sleep too well because their clock is delayed and by the time they get up to go to work, they are really sleep-deprived and quite naturally would rather sleep."

Allen also said that these people may have more trouble with jet lag or shift work. Most of the time, body "clocks" can be reset.

In recent years, sleep specialists have found that bright lights that mimic sunlight help people readjust their internal clocks. "When to give the lights and how much," said Allen, "depends on the individual patient."

In addition, he noted, for those who can manage it, taking a week off to catch up on sleep is a nice idea.

Timothy Monk, a psychiatrist and sleep specialist at the University of Pittsburgh, is focusing on sleep and astronauts.

Monk's proposal to monitor the sleep cycles of astronauts on a shuttle mission has been tentatively approved by the National Aeronautics and Space Administration and is scheduled for 1993.

Because the environmental cues that make the body's circadian clock run more or less on time are missing in the almost gravity-free space shuttle, scientists suspect that the rhythms will flatten. What this will do to alertness, mood, behavior and the general well-being of the astronauts has not been previously tested.

"When we move away from {short} missions where the astronauts can simply grit their teeth and get over whatever happens, it is something we'll have to know," Monk said.

He noted that there are the obvious problems with poor performance, which could have disastrous implications for safety. In addition, he said, there are "the more subtle things."

"We all know that when we're jet-lagged and working shifts, we're not as pleasant to live with. Clearly, that is important if there are six of you shut up in a little space station. You might be functioning at full strength, but if you're about to tear each other's eyes out in space, then that could be a problem," Monk noted.

Sleep researchers also are focusing on the relationship of sleep to creativity.

One British psychologist has shown that a lack of sleep can dim creativity and can turn an articulate, original thinker into something of a clod, with fixed ideas and a limited vocabulary.

James Horne of Loughborough University in Leicestershire, England, has devised some tests designed to measure creativity.

In a small study of two dozen British university students, he kept half of them awake for a 24-hour period and then asked them to perform certain tasks. One was to solve four problems involving patterns of colored cubes. None of the tasks was related to another, and each required a different approach.

The sleep-deprived group vainly tried to apply the same solution repeatedly.

They were also asked to list all the words they could think of beginning with certain letters, for which they would receive two cents per word. Most were significantly limited in their ability to do so.

Other tests measured their skill at dealing with the unexpected in otherwise normal situations.

"Think of an airline pilot," said Horne. "If he is sleep-deprived but doing something routine, it probably wouldn't matter. But if something he had never seen before were to occur, the original thinking that might be called for to get him out of a tight spot just wouldn't be there."

Horne has also found that, along with a drop in word fluency, sleepy people tended to sound flat and expressionless. In most people who sleep fewer than four hours, he said, "perseveration sets in," a mental inability to shift ideas or focus: in other words, a one-track mind.