Have you ever thought you were awake but were really asleep?

Watching a late late late night movie is a good example. It's something you really want to see, so you concentrate on keeping your eyes open and your attention fixed. Suddenly you realize a half hour has gone by and you haven't the foggiest idea of what was happening on the screen.

So you give up, click off the set and go to sleep. Or, really, back to sleep.

Your body clock was set to sleep. And no matter how you fought, something inside you clicked you off.

Lucky you weren't driving a car. Or watching a battery of complicated gauges in a nuclear power plant. Or flying a 747 . . .

No one has ventured to say for sure exactly what "human factors" cause any given accident, but scientists have pointed out that, for example, the accident at the Three Mile Island nuclear plant occurred at 4 a.m., in the middle of a night shift. On duty were workers who had only been on the 11 p.m.-to-7 a.m. shift for a few days and had been rotating shifts every week for the previous six weeks.

Researchers also have determined that characteristics of shift fatigue--when the conscious mind wants to be awake and thinks it is, but the internal clock commands sleep--include an inability to process complicated information and a strong tendency to ignore warnings or alarms.

Working around the clock was not practical until a century ago when Thomas Edison let his new lights shine into the darkness with the first power plant in history. Now about 20 million people work shifts in this country, and probably 60 million worldwide.

Yet it is only within the last few years that scientists began getting a real grasp on the consequences of shift work on the shift worker, and, inevitably, on the safety, effectiveness--and profits--of the industry that runs around the clock.

Chronobiologists, as the scholars of the new science of the rhythms of life are called, tell us that human biological clocks have to be reset by about an hour every day.

The planetary day, of course, is 24 hours, but human beings have a 25-hour cycle. Animals and birds vary, give or take an hour or so, but no live organism bigger than a bacterium or a virus fails to function on cycles roughly tuned to the planetary spin.

These cyclical rhythms--called circadian from circa-a-day--can be reset, but not at the simple desire of the organism or at the whim of a shift-planner. When resetting requires more than a few hours or when the organism (or person) gets older, the process gets more difficult, takes longer and costs more in general health.

Indeed, says Dr. Charles F. Ehret, a chronobiologist at the Argonne National Laboratories in Argonne, Ill., "shift schedules are usually designed by tradition, by the Navy or the Buffalo Steelworks in 1916 or by some kangaroo court where some know-it-all Joe decides."

Ehret has been researching circadian rhythms for around 35 years. In the beginning, he says, he was "a likely recipient" of the Golden Fleece Award that Sen. William Proxmire (D-Wis.) awards for the biggest waste of taxpayer's money. Ehret's project? The sex life of the paramecium. In fact, he wanted to know why that biology-1 protozoan only mated in the daytime.

And from that study has evolved one scary explanation of "the human factor" in the Three Mile Island accident, in a work by Ehret that has become a scientific classic, suggesting that shift fatigue may well have been a significant factor.

Even more, Ehret and other chronobiological and chrononeurological researchers have amassed a fund of knowledge that ought to add rhyme and reason--and health, safety and bottom-line cost-effectiveness--to the designing of work shifts.

Many of the same principles can be applied to jet lag, minimizing the time needed to reset circadian clocks that are thrown out-of-sync by international travel. Ehret, in fact, has written a book (with Lynne Waller Scanlon) encompassing the latest findings on body rhythms and travel: Overcoming Jet Lag (Berkley, $4.95).

Shift work is an even greater cause of the newly named disorder circadian desynchronosis--being out of time-sync. When you travel, says Ehret, even if you don't take any precautions, the life around you helps your body reset its clocks. But shift workers have a home life in one time frame and work in another.

Moreover, resetting the circadian clocks in a forward direction--from morning to afternoon to night--causes less stress on the body clock mechanism. It is the way the natural resetting moves.

Winding a watch backward will break the mainspring. Trying to reset the body's clock backward might result in a car accident, plane crash, or even a nuclear mishap. Yet many shift schedules work that way.

Studies also have shown that the longer the period between shift changes, the less the stress on the shifting organism, whether it be a transoceanic jet pilot or a laboratory rat.

Pilots and other far and fast traveling airline personnel have the worst of both modern major time disturbances: changing shifts and jet lag. Nevertheless, although many round-the-clock industries are factoring the new discoveries into shift designs, the Federal Aviation Administration has lagged.

Last week Ehret and other specialists from around the country testified at a house subcomittee hearing called by Rep. Albert Gore (D-Tenn.) to pool and eventually publicize the latest scientific data on minimizing the effects of shifting.

"We are wondering," says Ehret, "whether in fact we haven't reached a position where we know so much about a particular scientific discipline that we can start to do something about it . . . instead of letting it spread through a slow grapevine."

Here are some of the discoveries of the past few years:

* All sorts of human functions are affected by circadian cycles--probably many more than have been identified. Literally billions of cells must be synchronized. For example, certain hormones are turned on an hour or so before your alarm clock goes off so you'll be truly awake when it is time to get up.

Various enzymes are programmed to go on and off all day; nerve cells fire according to the inner clock to affect mood and energy. Body temperature is raised or lowered; metabolism is speeded up or slowed down.

The cycle that dictates sleep/wake is almost trivial compared to the complexity and elegance of the overall circadian system.

* Then there is what Ehret calls "a laundry list" of things that affect one or more circadian cycles. At first, it was believed that it was all just a matter of day and night--light and dark. Science now has added more "zeitgebers," literally, "time-givers." These can include foods, drugs, exercise and social cues. (Is everybody else awake when your body is telling you you're wiped out?)

* The "zeitgebers" can have different effects depending on their quality--carbohydrate versus protein or an anti-hypertensive drug versus the family of methylxanthines that includes coffee, tea and chocolate. New studies are trying to identify chronoactivity in medications and how to determine the best time to take them.

* The same "zeitgebers" can have opposite effects, depending on what point in the circadian cycle they are consumed or experienced. For example, beverages high in caffeine will tend to shift you one way when taken in the morning (as perceived by your inner clocks) and will shift you the opposite way when taken at night. (They are useful in the jet-lag prevention program.)

However, Ehret has found, if caffeine is consumed "at British tea-time" it appears to smooth out any minor synchrony problems and is, in fact, "probably good for you in modest quantities." On the other hand, even if all the time-givers were right, but you nibbled around the clock, your synchrony would be seriously disrupted.

Ehret suggests that "a possible cause of senility is putting zeitgebers into the wrong pattern in nursing homes, for example." Animal studies suggest that desynchronosis can shorten life spans and reduce resistence even to cancer, but this work is very preliminary.

To help shift workers, Ehret and his colleagues at Argonne have prepared an at-a-glance chart to guide intake of food and exercise for workers on various shifts. A similar chart for jet-lag prevention, reprinted in the book, guides travelers.

For more information and copies of charts write Dr. Charles F. Ehret, Biological and Medical Research, Argonne National Laboratory, Argonne, Ill. 60439.