A study on sleep found that people’s accuracy on working-memory tasks dropped by about 15 percent while they pulled an all-nighter. Sleep helps your brain consolidate information, so without that recovery time, you’re unable to file away important data.

YOUR WAISTLINE EXPANDS

“Some research has suggested that sleep restriction over many years may affect metabolism, increasing the risk of obesity and type-2 diabetes,” says Siobhan Banks, Ph.D., an assistant professor of psychiatry at the University of Pennsylvania. In fact, a 2007 Canadian study found that people who sleep only 5 or 6 hours a night increase their likelihood of being overweight by 69 percent, compared with those who habitually sleep 7 or 8 hours.

YOU GET SICKER

University of Chicago researchers found that antibodies in sleep-deprived people who’d received a vaccine were about 50 percent weaker than those in well-rested people. Wake up early for that flu shot and it may not protect you so well. Harvard researchers also found that blood levels of C-reactive protein, a marker of heart-disease risk, can spike if you skip the sheets for just a few hours.

YOUR CANCER RISK RISES

During sleep your body produces melatonin, a hormone that helps regulate your sleep cycle. Interrupt sleep and you interrupt melatonin synthesis, which can be a problem. A 2007 University of Texas study review concluded that not only does melatonin detoxify harmful, cancer-causing free radicals, but in doing so it actually creates more antioxidants. Melatonin may also boost the effectiveness of vitamin C, another antioxidant. A good snooze could goose your morning OJ’s potency.

Sleep is no different from diet or exercise. If you eat 10 percent more calories a day, it can add a stone in weight over year. But we fail to understand that sleeping 10 percent less carries a similar risk for weight gain. In fact, people who sleep five or fewer hours a night are one third more likely to gain weight than those who get a good seven hours of slumber, according to the International Journal of Epidemiology.

And that’s just for starters. It’s best for our body to cycle through the five sleep stages (see below) four or five times a night: The first four stages are key to maintaining healthy metabolism, learning and memory; the fifth (rapid eye movement sleep, or RE M) is important for regulating mood and forming emotional memories. Miss a cycle or two and our immune system, heart health, brain function and more can suffer.

BEYOND TIRED

Droopy eyelids and low energy are the least of your worries when you’re sleep-deprived. If you aren’t getting the seven to eight hours that’s ideal, you canseriously compromise your health.

MORE THAN EIGHT HOURS

Unfortunately, you can have too much of a good thing. Regularly amassing more than eight hours a night disrupts blood sugar levels, which makes type 2 diabetes a concern, according to findings in the journal Diabetes Care. And sleeping more than nine hours is linked to an increased risk of dying due to any cause, say researchers at the University of California at San Diego and elsewhere, possibly because other health conditions that cause fatigue are a factor.

FEWER THAN SEVEN HOURS

You’re three times more likely to catch a cold if you sleep fewer than seven hours per night than if you get eight, possibly because sleep helps regulate the body’s response to infection. Weight gain also becomes a worry: We produce more of the appetitepromoting hormone ghrelin and less of the satietyproducing hormone leptin when we’re low on sleep.

6 HOURS OR FEWER

You may think you function fine on this little sleep, but snoozing six hours or fewer a night for a period of only two weeks will impair your memory, reaction time and general cognition in the same way that staying awake for up to 48 hours straight would, according to a study from the University of Pennsylvania School of Medicine in Philadelphia.

THE FIVE STAGES OF SLEEP

The invention of the electroencephalograph allowed scientists to study sleep in ways that were not previously possible. During the 1950s, a gradate student named Eugene Aserinsky used this tool to discover what is known today as RE M sleep. Further studies of human sleep have demonstrated that sleep progresses through a series of stages in which different brain wave patterns are displayed.

THERE ARE TWO MAIN TYPES OF SLEEP

Non-Rapid Eye Movement (NREM) Sleep (also known as quiet sleep

Rapid Eye Movement (REM) Sleep (also known as active sleep or paradoxical sleep)

THE BEGINNINGS OF SLEEP

During the earliest phases of sleep, you are still relatively awake and alert. The brain produces what are known as beta waves, which are small and fast. As the brain begins to relax and slow down, slower waves known as alpha waves are produced. During this time when you are not quite asleep, you may experience strange and extremely vivid sensations known as hypnagogic hallucinations. Common examples of this phenomenon include feeling like you are falling or hearing someone call your name. another very common event during this period is known as a myoclonic jerk. If you’ve ever startled suddenly for seemingly no reason at all, then you have experienced this odd phenomenon. While it may seem unusual, these myoclonic jerks are actually quite common.

STAGE ONE

Stage 1 is the beginning of the sleep cycle, and is a relatively light stage of sleep. Stage 1 can be considered a transition period between wakefulness and sleep. In Stage 1, the brain produces high amplitude theta waves, which are very slow brain waves. This period of sleep lasts only a brief time (around 5-10 minutes). If you awaken someone during this stage, they might report that they weren’t really asleep.

STAGE TWO

Stage 2 is the second stage of sleep and lasts for approximately 20 minutes. The brain begins to produce bursts of rapid, rhythmic brain wave activity known as sleep spindles. body temperature starts to decrease and heart rate begins to slow.

STAGE THREE

Deep, slow brain waves known as delta waves begin to emerge during stage 3 sleep. Stage 3 is a transitional period between light sleep and a very deep sleep.

STAGE FOUR

Stage 4 is sometimes referred to as delta sleep because of the slow brain waves known as delta waves that occur during this time. Stage 4 is a deep sleep that lasts for approximately 30 minutes. bed-wetting and sleepwalking are most likely to occur at the end of stage 4 sleep.

STAGE FIVE

Most dreaming occurs during the fi fth stage of sleep, known as rapid eye movement (REM) sleep. REM sleep is characterised by eye movement, increased respiration rate and increased brain activity. REM sleep is also referred to as paradoxical sleep because while the brain and other body systems become more active, muscles become more relaxed. Dreaming occurs due because of increased brain activity, but voluntary muscles become paralyzed.

THE SEQUENCE OF SLEEP STAGES

It is important to realise, however, that sleep does not progress through these stages in sequence. Sleep begins in stage 1 and progresses into stages 2, 3 and 4. after stage 4 sleep, stage 3 and then stage 2 sleep are repeated before entering REM sleep. Once REM sleep is over, the body usually returns to stage 2 sleep. Sleep cycles through these stages approximately four or fi ve times throughout the night. on average, we enter the REM stage approximately 90 minutes after falling asleep. The first cycle of REM sleep might last only a short amount of time, but each cycle becomes longer. REM sleep can last up to an hour as sleep progresses.