You've heard you need eight hours of shut-eye every night, but you swear you're good with six. Here's how to tell how much sleep you really need.

Yet again, you spent the night tossing and turning, and now you face the day feeling tired and cranky. What's going on inside your body isn't great, either, explains Tara Maltman-Just, pharmacist and executive clinician at Vitality Integrative Medicine in Winnipeg.

"Your sleep-wake cycle affects a lot more than just your energy throu­­­­ghout the day," she says. "It affects your immune function, your blood sugar. In fact, too much sleep or too little sleep can decrease life expectancy." We've all heard that adults need seven to eight hours of sleep each night, but that's an average—some of us need more, while others are fine with less. How can you find your perfect number on the sleep spectrum?

Add more hours
The first clue may be how you talk about it. "When people say they can ‘get away with' a certain number of hours of sleep, they're admitting they're not getting enough," says Dr. Charles Samuels, founder and medical director at the Centre for Sleep and Human Performance in Calgary. "It means they know how much sleep they need to feel well rested on a daily basis, but they're not getting it."

Another sign: feeling bad-tempered or foggy-brained. Patience and clear thinking are often the first things to go in sleep-deprived individuals. "People become more irritable and inattentive," says Dr. Samuels. "The less rested you are, the less creative your thinking can be."

And if you've got the urge to eat, you might need a siesta, not snacks. "As we shorten the sleep we need, it increases the appetite for calorie-dense foods," explains Dr. Samuels. "It contributes to the onset of weight gain and poor weight control."

Too much of a good thing
On the flip side, if you regularly spend the bulk of your free time snoozing, you might be sleeping more than you should. Oversleeping can be an indication of a host of other health problems—hypo­thyroidism and mood conditions such as depression, chronic fatigue and fibro­myalgia—all of which require treatment.

Of course, there are times when additional sleep is necessary—like when your body is recovering from illness or surgery. "In those cases, it makes sense to have more sleep," says Maltman-Just, "to allow the body to rest and heal." Bottom line? If you constantly feel the need to oversleep but aren't sure why, talk to your doctor to address any underlying issues.

The trick is to find you­r own happy medium—the number of hours of sleep needed to wake up rested and refreshed. If a restful vacation helped you realize you need eight hours of shut-eye per night and you're only clocking five, try these simple strategies to increase your total number of hours of zzz's.

- Keep a sleep diary
Recording your sleeping and waking times in a diary—as well as your meals, exercise habits and caffeine and alcohol intake—is a helpful tool for improving sleep quality, according to Dr. Harvey Moldofsky, director of research at the Centre for Sleep and Chronobiology and president of the Toronto Psychiatric Research Foundation. The logic: A diary can provide clues as to why you might be undersleeping and, by extension, potential lifestyle changes to promote better sleep.

- Think weekly—not daily—totals
If you're not getting your ideal number of hours of sleep every night, don't fret. "We always go on total number of hours of sleep per week," says Dr. Samuels. "People need 50 to 60 hours of sleep per week; try to get close to that." His suggestion: Keep your weekly sleep debt down with strategic naps, sleep-ins of up to one hour and consistent bedtimes throughout the week.

- Aim to sleep later
Although you might be tempted to go to bed earlier to gain extra hours of slumber, you'll be better served by adding time to the end of your nightly sleep cycle whenever possible. "The way the brain likes to sleep is going forward," notes Dr. Samuels. "So maintain your bedtime—maybe turn in half an hour earlier—and then add to the other end, up to an extra hour." If your work schedule prevents sleeping in, try to get an extra hour of sleep on Saturdays and Sundays. If your sleep is so poor that you routinely get less than 50 to 60 hours a week, see your doctor.