Go on then, just a small nightcap to help me to sleep ... The trouble is, nightcaps aren’t usually small and they don’t help you to sleep. In fact, they make sleep worse. Research shows that although one person in seven — and more than a quarter of insomniacs — uses alcohol as an aid to sleep, it actually disrupts sleep, even at comparatively low levels. The more we drink, the more tired we end up.

A recent survey conducted for the Department of Health found that 58 per cent of people were unaware that drinking alcohol could disturb a good night’s sleep. The problem isn’t just ignorance — the evidence of our own experience seems to defy the research findings.

How can alcohol be bad for sleep when it so clearly makes me nod off?
There is a real difference between making you go to sleep and sleeping well. Studies have shown that alcohol does, indeed, make you go to sleep more quickly, at least if it’s not drunk in excess — it has a natural sedative effect. However, once you are asleep it disrupts what scientists call “sleep architecture” — the pattern of sleep and brain waves that leaves you feeling refreshed in the morning. We need the right balance of REM sleep (dreaming sleep) and non-REM sleep (including deep sleep), and alcohol disturbs this.

“Alcohol can mean that sleep is no longer refreshing, because the brain can’t perform the normal restorative job it does during the night,” says Jessica Alexander, of the Sleep Council, which provides information about sleep and health.

But I don’t feel that I haven’t slept well after a drink . . .
The problem isn’t always obvious sleeplessness. You may feel that you slept well, especially for the first half of the night.

During the first portion of sleep, alcohol drunk within an hour of bedtime increases the proportion of non-REM sleep and decreases the proportion of REM sleep, but it doesn’t greatly disrupt sleeping patterns. The problems come mainly in the second half of sleep, when REM falls off again and sleep becomes more disturbed. People taking part in sleep experiments have reported that they sleep more superficially a few hours into their sleep — and it has been observed that their limb movements increase, their temperature fluctuates, they are more dehydrated and are more likely to awaken from dreams. This may be partly because, in the second half of the night, your body is suffering alcohol-withdrawal symptoms once it has processed the alcohol you put in your bloodstream before going to sleep. It may also be a side-effect of some of the toxins produced by the breakdown of alcohol.

But I’m usually not drunk when I go to sleep: I only have a small nightcap.
Even small amounts of alcohol can affect the body during sleep. It slows breathing, makes the lining of the throat more swollen, and reduces the tone of muscles in the airway — making breathing problems such as snoring and sleep apnoea more likely. Oxygen deprivation is bad for your sleep and your health generally.

Studies show that a “moderate” dose of alcohol — say three bottles of beer or three glasses of wine — consumed as much as six hours before bedtime can increase wakefulness during the second half of sleep. On the other hand, if it genuinely is just one drink you have before you go to bed— say, a small glass of wine — then there is little evidence that there will be a serious effect on your sleep quality, and it may help you to nod off. In fact, some scientists believe that this is a good approach for insomniacs (but not the population in general).

The problem is that if we try the tactic again and again, we become increasingly immune to alcohol’s sleep-inducing qualities. That means we keep on having to increase the dose to have the required effect — and a small glass of wine can end up being a pint-glass full. That will certainly impair your sleep, and leave you feeling wretched the next day.

After a heavy drinking session, I’m comatose and can’t be woken — so I’m clearly sound asleep.
If you’ve had too much to drink, you will have been taken over by the sedative effect of alcohol, which suppresses brain activity, rather than by true sleep, which is an active brain process.

Top tip for sleeping: take a nap
Naps are good for you — in moderation. Research suggests that napping for 20 minutes can help to refresh the mind, improve overall alertness, boost mood and increase productivity, although napping for longer than 30 minutes can make you feel more tired when you wake up. In a six-year study of Greek adults, researchers found that men who took naps at least three times a week had a 37 per cent lower risk of heart-related death. So unplug your BlackBerry, turn off the computer and pop on that eye mask.