1. It's impossible to tell if someone is really awake without close medical supervision. People can take cat naps with their eyes open without even being aware of it.

2. Dreams, once thought to occur only during REM (Rapid Eye Movement) sleep, also occur (but to a lesser extent) in non-REM sleep phases. It's possible there may not be a single moment of our sleep when we are actually dreamless.

3. REM dreams are characterised by bizarre plots, but non-REM dreams are repetitive and thought-like, with little imagery - obsessively returning to a suspicion you left your mobile phone somewhere, for example.

4. Humans sleep on average around three hours less than other primates like chimps, rhesus monkeys, squirrel monkeys and baboons, all of whom sleep for 10 hours.

5. Ducks at risk of attack by predators are able to balance the need for sleep and survival, keeping one half of the brain awake while the other slips into sleep mode.