HOW well people function when they are awake depends a great deal on whether they get enough sleep, says the National Institutes of Health in the United States.

It says on its website: "Studies show that a good night's sleep improves learning.

"Whether you're learning maths, how to play the piano, how to perfect your golf swing or how to drive a car, sleep helps enhance your learning and problem-solving skills."

This is because it is during sleep that the brain forms new pathways that help people learn. Not getting enough sleep can alter your brain pattern and make it more difficult for you to make decisions, solve problems or cope with change.

You might also "feel frustrated, cranky or worried in social situations".

It adds: "Sleep deficiency is linked to many chronic health problems, including heart disease, kidney disease, high blood pressure, diabetes, stroke, obesity, and depression."

This is because it is during sleep that the body releases the hormones that boost muscle mass, repair cells and tissues and regulate puberty and fertility.

But not everyone needs the same amount of sleep, or even needs to sleep at the same time.

Young children and older people tend to sleep and wake up earlier, while teenagers fall asleep later at night.

And it is not just teenagers' social habits that account for this. Their bodies release the hormone melatonin, which triggers sleep, later, so "it's natural for many teens to prefer later bedtimes at night and sleep later in the morning than adults".

One sign that you are not getting enough sleep is if you tend to sleep longer on your day off.