It's important to know, as any less than five hours is a real tipping point for bad health.

Sleep is extremely important for us all, it helps rejuvenate, rest and restore both the body and mind - but quite why the exact minimum number of hours spent in the sack matters is unclear.
But it does.
Researchers say catching enough Zs may drastically reduce the chances of several health problems in the over-50s.
A PLoS Medicine study tracked the health and sleep of around 8,000 UK civil servants, asking each of them: 'How many hours of sleep do you have on an average weeknight?'
Some also had their sleep monitored via a wrist-watch.