Shakespeare described sleep as "the chief nourisher in life's feast", acknowledging that for most of us deep rest is necessary for keeping body, mind and spirit in good form.

What if restless nights are normal instead of the exception?

Sleep deprivation isn't just bad for us, it's also bad for people around us: tens of thousands of traffic accidents are attributed to sleepy drivers every year.

But the average night's sleep in the UK now averages about 90 minutes less than it did in the 1920s, when it was nine hours a night.

What is sleep?

Sleep is made up of different stages. The ten-minute period of light sleep (drifting off from wakefulness) is called Stage One. Stage Two is deeper and lasts about 20 minutes. Stages Three and Four come after this, and are types of deep sleep.

Deep sleep is the part of sleep that our body and brain needs to recover from the day. It's sometimes called delta sleep, after the delta waves the brain generates. We don't dream during this time. After about 90 minutes of deep sleep, rapid eye movement (REM) sleep begins.

During a normal night, people usually cycle through the various stages several times. Problems arise when the pattern of cycles is broken.

Are we getting enough?

Those of us who don't have the luxury of an afternoon nap still tend to get sleepy at about this time. Interestingly, people who increase their night-time sleep find that this afternoon 'dip' disappears. A ten-minute nap at lunchtime is just as effective.

Sleep problems, including dozing off, can be a symptom of many other conditions, from problems with the thyroid gland to depression - and even occasionally rare sleep disorders such as narcolepsy. So it's important to speak with your GP if you're currently experiencing insomnia or another sleep problem, since it could be indicative of another condition.

Sleep and mood are interlinked in intriguing ways that are still not understood. But a satisfying sleep, like a satisfying meal, can leave one happy and content - with room perhaps for just a little more.

Remember to use your bedroom primarily for sleeping, making it as comfortable, airy and dark as possible.