But a 2014 study by psychology professor Kristi Erdal argues that the amount of sleep you think you get may be more important than how much you actually get.
Erdal's research took 21 undergraduate students from Colorado College, gave them a five minute lesson on what constituted good sleep and bad sleep, and got them to report on how much they had slept the night before.
Students were connected to Biopac equipment, which they were told would measure their pulse, heart rate and brain frequency (only brain frequency was actually measured).
Findings showed that the students who were told they had slept well performed far better in cognitive tests than those who were told they had slept badly.
When participants were informed that they had experienced below-average sleep quality the night before, they tended to perform worse on the test, regardless of how well they felt they had slept.
The observed pattern of cognitive functioning is consistent with what one might observe if participants had actually experienced a poor night’s sleep.
So...believing you've slept well is enough?
Erdal calls it "classical conditioning" which may also be why another study concluded that drinking a coffee placebo has a similar effect on brain function and concentration as a cup of real caffeinated coffee.
We should take the findings with a pinch of salt, though.
The NHS stresses that sleep deprivation can make you prone to serious medical conditions like obesity, heart disease, diabetes and high blood pressure, and while it may vary from person-to-person, eight hours of sleep a night is the general rule for optimum health.