Humans wake up more than 100 times during the night - without actually realising.
Humans wake up more than 100 times during the night without realising
The stress hormone noradrenaline is repeatedly released into the brain during sleep so its activity reaches the same level as when people are awake.
The process is vital to how humans store memories from the day but happens so briefly that it is not noticed.
Assistant Professor Celia Kjaerby, from the University of Copenhagen in Denmark, said: "You may think that sleep is a constant state that you are in and you wake up.
"But there is a lot more to sleep than meets the eye. We have learned that noradrenaline causes you to wake up more than 100 times a night. And that is during perfectly normal sleep.
"Our study suggests that short-term awakenings are a natural part of sleep phases related to memory. It may even mean that you have slept really well."