BIS FOR BABIES The little 'uns should always be laid to sleep on their backs.

This is the surest way to protect against cot death. A baby laid to sleep on its front is more likely to suffocate or become overheated. On its back with its feet against the bottom of the cot it is less likely to slip down under the covers. Since the Back to Sleep campaign was launched in the early 1990s, cot deaths have fallen from about 1,000 a year to 300 in the UK.

CIS FOR CIRCADIAN RHYTHM generated and maintained by the body's biological clock. It affects temperature, blood pressure, sleep and wakefulness, mental performance, and the synthesis of hormones. Trying to sleep at the wrong phase of the circadian cycle will usually lead to a disturbed night with frequent awakenings resulting in the exhaustion seen in jet lag and among shift workers.

DIS FOR SLEEP DEPRIVATIONAnyone who has gone for two nights without sleep will know what this means. The siren call of slumber beckons irresistibly. Keeping victims awake is a well-known form of torture, to reduce their defences and soften them up for questioning. The human organism cannot do without sleep and prolonged deprivation results in confusion, hallucinations and delirium.

EIS FOR EXERCISE The friend of sleep. Moving your body during the day prepares it for rest at night as well as easing stress, the enemy of sleep.

Thirty minutes of exercise a day is recommended. Climb the stairs instead of taking the lift, walk the dog, chase the children, do the housework. It doesn't have to involve going for the burn in the gym.

FIS FOR FATIGUE Doctors of the old school had an acronym for their most difficult patients - TATT (tired all the time). Most adults need seven or eight hours sleep a night, though some manage on much less and a few sleep as long as 10 hours. The relation between sleep and fatigue is complex, however. Fatigue is often the product of something else that is wrong - such as anxiety or depression.

GIS FOR GETTING UP Not just in the morning, which is always hard, but at night, when you can't sleep. Never lie in bed awake for more than 20 minutes, sleep specialists say. Switch the light on and read or get up, go and sit somewhere dark and warm and comfortable with your eyes closed. When you start to feel sleepy, close your book and go back to bed.

HIS FOR HIBERNATION, which may seem an attractive option in the dark days of winter but is not open to human beings.

Hedgehogs and doormice hibernate in Ireland - a survival strategy that has evolved to see them through the winter food shortage. During hibernation, animals lower their metabolism dramatically and live off their body fat - waking weeks or months later thin and depleted.

IIS FOR INSOMNIA Ireland is a nation of insomniacs with almost two thirds of the population claiming not to get enough sleep. However, experts say the pre-occupation with sleep loss is itself becoming a medical problem. The greatest enemy of sleep is worry about not getting enough of it. Most people who lose sleep will be able to recover it in 24 hours and be able to cope in the meantime. Overcoming the fear of not being able to cope is a way to conquer insomnia.

JIS FOR JET LAGCrossing time zones disrupts the sleep-wake cycle leaving longhaul travellers red-eyed and moody. It is an effect of the light that regulates production of melatonin, a hormone made in the pineal gland in the brain. Melatonin is secreted at night and in countries where it is available as a drug it is used as a treatment for jet lag and as a sleeping pill. But evidence is lacking for its long-term safety.

KIS FOR KILOCALORIES, a measure of the amount we eat. Too little and we will spend the night thrashing about, too restless to sleep because our bodies have not had enough to meet their energy needs. Too much and we will toss and turn uncomfortably. A meal of protein and vegetables should include some carbohydrate - pasta, potatoes, rice or bread - which have a calming effect on the body.

LIS FOR LIGHT Electricity is to blame for our chronic state of sleep deprivation.

Before Edison invented the light bulb, people woke with the dawn and retired at nightfall. Now in cities across the world night is turned into day and sleep is optional. Light also affects mood, as seen in seasonal affective disorder (Sad), which increases in winter and decreases with the longer days of summer.

MIS FOR MODAFINIL, a stimulant drug that counters sleepiness developed for the treatment of narcolepsy (see below). It attracted the attention of the military after tests showed it boosted memory and reaction times in normal volunteers. A trial in helicopter pilots flying on simulators after being sleep deprived showed it boosted performance. Its capacity to keep people awake and alert for long periods without the "rebound" effect of amphetamines is seen as a key advance.

NIS FOR NARCOLEPSY, a rare sleep disorder whose sufferers live every day as though the previous 48 hours have been sleepless. Sufferers may fall asleep during a meal, at the wheel of a car or in midconversation. It is thought to be an immune disorder which affects production of the hormone hypocretin which regulates sleep.

OIS FOR OBESITY The less children sleep, the fatter they become, research has shown. The finding is counter-intuitive - more activity ought to equal less weight - but laboratory studies have shown that people who sleep less have lower levels of the hormones leptin and ghrelin which regulate hunger and appetite. Hectic parental lifestyles and lax bedtimes may be contributing to the explosion in obesity.

PIS FOR PROSTATE PROBLEMS, a common cause of sleep disruption in men.

Half of men aged over 40 have an enlarged prostate, the gland below the bladder that produces the seminal fluid in which the sperm swim. Symptoms include nocturia - the need to get up in the night to pee. For some men, who return to bed and fall straight asleep again, this is no more than a minor inconvenience but for others it can induce insomnia.

QIS FOR QUALITY How you sleep is as important as how much. A few hours of good quality sleep is better than eight hours of tossing and turning.

Although alcohol helps induce sleep, it can disrupt deeper sleep cycles and lead to waking - with a sandpaper mouth and throbbing head. It is possible that early waking may also be a symptom of depression.

RIS FOR REMEDIES Those aids to sleep for people who have trouble dropping off at night. The herbs valerian, lemon balm and hops are all reputed to induce sleep and may hold more appeal than conventional sleeping pills. A warm drink can be soothing and relaxing but avoid those containing caffeine - tea and coffee - and substitute herbal tea or a milky drink such as cocoa. Sex is also an effective soporific.

SIS FOR SIESTA, which is going out of fashion in the countries of the southern Mediterranean where a leisurely lunch and a snooze were seen for decades as the best way of coping with the midday heat.

Now new research from the Harvard School of Public Health published this week suggests it has medical benefits. An afternoon nap can cut the risk of death from heart disease among men and women by a third.

TIS FOR THATCHER, Margaret, the former prime minister of Britain fabled for her capacity to manage the affairs of state on four hours sleep a night, "provided that about one day a week you have a night when you can have longer", as she told a BBC interviewer. Her regular nightcap was a large whiskey, and often several, which went against medical orthodoxy that says alcohol helps sleep come but leads to a disturbed night. Thatcher never showed the effects the next day.

UIS FOR UNSOCIAL HOURS Experts say the 24-hour society is shortening lives and harming health because of the havoc it plays with our biological clock.

One in five workers in urban societies is now working outside of normal office hours and paying the price in terms of an increased risk of accidents and of heart disease.

VIS FOR VAMPIRES, which sleep by day and hunt by night.

After a night's activity, they will sense the dawn approaching even if they cannot see the light and start to feel tired. They are capable of dreaming like humans, but their limbs do not move and their eyes do not show the characteristic rapid flickering. Once night comes round again they will wake and show no inclination to sleep until dawn.

WIS FOR SLEEP-WALKING Jennifer Aniston, former Friends star, is reported to wander about her Los Angeles mansion at night and then wake up not knowing where she is. The behaviour is known to be linked with emotional disturbance and Aniston, 37, who was married to Brad Pitt, is said to be upset at the break-up of her relationship with US comedian Vince Vaughn.

XIS FOR X-RATED DREAMS, which enliven the sleeping hours of millions. Sex is a favourite theme though often disguised as dreams of flying or falling. As Freud explained, the meaning of dreams is never obvious, least of all to the dreamer.

Which is why it is never wise to discuss your dreams in public - you never know what you may be giving away.

YIS FOR YOUTH and the legendary need for sleep of teenagers. Chronic sleep deprivation is now a recognised medical problem among young people. They need more sleep than adults because they are growing, which takes energy, but often get less. As we get older we need less sleep.

The elderly, who have time to sleep, often have the greatest difficulty in staying in bed.

ZIS FOR ZZZ - which is the object of the exercise.