"I can't sleep." This complaint is far too common - it is estimated that more than 40 million Americans regularly have serious sleep problems.

But what is insomnia?

The National Institutes of Health in the US defines insomnia as " ...complaints of disturbed sleep in the presence of adequate opportunity and circumstance for sleep".

In other words, you have time and there's your bed, but you still can't seem to fall asleep. Maybe you do fall asleep but wake up far too early.

To find an insomnia cure it is helpful to get a rough idea what is the basic cause. The occasional night with bad sleep is not the same as insomnia.

Everyone will have the occasional sleepless night. Maybe we stay up watching a favourite football game, drinking beer, or we are waiting for a late flight, sitting in a café drinking coffee.

Simply disrupting your normal sleep pattern can cause temporary sleep difficulties. That's not a problem, it's a temporary setback.

More severe and frequent problems sleeping need to be looked at more carefully. Try to reason through why you cannot sleep - is there any functional or physical reason that prevents you from falling asleep, or keeps waking you up?

Things like late night coffee drinking, working shifts, maybe taking certain medication? Or is your house under the flight path of late night flights?

If you're overweight the reason might be you have sleep apnea - what this means your body doesn't get enough air because you snore!

When this happens your reflexes will wake you up so you can breathe.

Still talking about physical reasons for sleep disorders, conditions like diabetes and high blood pressure might affect your sleep, causing insomnia and sleeplessness.

Having said this, it is more common to find the causes of insomnia in the way your mind works.

If you are worried about money, for example, or in an unhappy relationship, your mind will churn negative thoughts around instead of the calming and relaxing thinking that primes you for sleep.

This is the reason experts in sleep medicine always advocate the "simple" method of learning to "wind down" before bedtime.

Yoga, meditation, quitness and darkness - all things that help to slow down your mind and make it easier to fall asleep.

Simple things like these may well be all you need to cure your insomnia, unless your sleep disorder is really severe.