You've whittled down your To-Do list, taken a course in Time Management and organized your priorities. But there still are never enough hours in the day.

"If you could sleep just one hour less than you've been sleeping," says writer Everett Mattlin, "you could gain a measure of peace and quiet in the morning to clear up matters in the office or at home, or some time in the evening to read that long-neglected book or pursue a new interest."

An 8-hour-a-night sleeper who cuts back to 6 1/2 hours would gain "an additional 547 hours a year -- the equivalent of an extra month of waking hours. That's like adding a year to every 12 'years you will live."

But everyone needs eight hours of sleep to function -- right?

Wrong, claims Mattlin, who says the "eight-hour myth" has been drummed into our heads by everyone from parents to insurance companies to Alfred the Great (the 9th-century Saxon king who proclaimed "Eight hours for work, eight hours for play and eight hours for sleep").

"There is no norm of seven or eight hours that we must all conform to," he says, citing recognized superachievers and short sleepers like Thomas Edison (4 hours), Isaac Asimov (5 to 6 hours) and Edward Koch (5 1/2 hours).

Although he doesn't suggest we emulate the 70-year-old retired nurse who sleeps just one hour each night, he says "most people could probably reduce their sleep time an hour or two without hurting their body or mind."

The 52-year-old free-lance writer (and founding editor of Gentleman's Quarterly) started investigating sleep reduction several years ago after he noticed that "a lot of successful business people are early risers.

"I myself was a confirmed 8-hour sleeper, and became intrigued by these very successful, driven, highly-motivated early-risers. I looked upon them as alien creatures."

After extensive research convinced him that "cutting back (on sleep) does not hurt your body or mind," he began slicing half hours off his sleep schedule.

"Going from 8 to 7 1/2 was no problem," he recalls.

"Seven wasn't so bad. At 6 1/2 I began to feel it. It took me a couple of weeks to feel comfortable. At 6 I couldn't function at work, so I went back to 6 1/2 until I was comfortable. Then I tried 6 again.

"It seemed to work and I've been very pleased with the results."

Just as a dieter can learn to eat less, Mattlin claims, nearly anyone can learn to sleep less -- if they are motivated. "You won't want to leave your bed earlier in the morning," he notes, "if there's no compelling reason."

Mattlin used his extra hours to write "Sleep Less, Live More" (J. B. Lippincott Co., 176 pp., $9,95) and now catches up on reading in his "spared time." He warns, however, that sleep reduction takes adjustment and will probably entail periods of discomfort.

"You're reorganizing a pattern of life. People shouldn't push too far past a comfort level -- 6 hours is probably the stopping point.

"And I don't want to tell people to drive themselves -- to sleep less so they can work more and more.I'm not trying to send people to an early grave. Freeing up time is meant to reduce pressures, not fill up every additional minute with still more things to do."

Although Mattlin seems to have done extensive, thoughtful research, some sleep experts disagree with his conclusions. "We can't will ourselves to sleep less anymore than we can will ourselves to grow a third leg," says University Florida sleep expert Wilse Webb.

"It is possible to sleep less, but you're going to be miserable for a long time," says Dartmouth College's sleep clinic director Peter Hauri. "You can't look at people who are naturally short sleepers and say because they do it, everyone else can, too."

Psychiatrist Gordon Globus, however, who did sleep research at the University of California at Irvine says, "Our bodies may not need anywhere near as much sleep as we actually obtain. I know of no reason to think (reducing sleep by 1 to 1 1/2 hours a night) is harmful."

Persons with any medical or psychological disabilities should consult their physician before trying a sleep-reduction program, says Dr. J. Christian Cillin of NIMH's unit on sleep studies.

"People who have to maintain more than usual alertness -- such as truck drivers or air-traffic controllers -- probably shouldn't try it," along with alcoholics, says Dr. Gillin.

"In terms of the average, healthy person it's probably okay. In general, there's probably no major detriment to a few night's sleep loss."

"Why not develop the habit of sleeping less?" asks New York sleep therapist Abraham Weinberg. "After all, there is nothing to lose except a little sleep."

Mattlin's sleep-reduction program includes these steps:

1. Defin exactly what you want to do with your gained time. Since motivation is vital, it's helpful to spend the extra time doing something you wouldn't have been able to do otherwise.

2. Start only when you're rested.

3. Begin by sleeping two weeks at your regular level. Keep a daily log of how many hours you sleep and how you feel each day.

4. Stick to a regular schedule. But make allowances in times of stress -- someone with a bad cold needs more rest.

5. Reduce your sleep time gradually. Cut down in half-hour segments, giving your body, mind and internal clocks at least three weeks to adjust to each level.

6. Be prepared for discomfort and don't push too hard. If after 10 days to two weeks you're not adjusting well, give up that level, return to the last comfortable level and try again later. If after three weeks any reduction of your usual sleep time leaves you feeling upset, the program probably isn't for you.