Sleep inertia is the groggy state we sometimes experience upon waking. It is most noteworthy during the first 15 to 30 minutes after we wake up. During this state our cognitive functioning dips – something which improves once we have been awake for a while.
In an excellent review of the topic published in the Nature And Science Of Sleep in 2019, authors Cassie Hilditch and Andrew McHill point out that while sleep inertia can occur in the absence of risk factors, it is worst when waking occurs during the ‘biological night’; after we have previously missed out on sleep; and that it might also be linked to deeper sleep. Indeed, it is sometimes recommended that naps are limited to 15-20 minutes in order to reduce the likelihood of sleep inertia occurring. Research continues on the neurophysiology of this state, but certain differences in brain activity and connectivity have been flagged in the awake brain before as compared to after sleep.
Sleep inertia could also reflect adenosine being inefficiently cleared from the brain during sleep – as caffeine has been found to be effective in reducing sleep inertia. We do not know whether sleep inertia is associated with certain advantages, but one intriguing hypothesis flagged in the aforementioned review is that this process could encourage us to stay asleep rather than become fully awake following unwanted disruptions to our sleep.
Sleep inertia can have real-life consequences (it increase the risks of accidents when driving, for example) and is the reason why we should always give ourselves time to wake up fully after sleep and before getting on with the day. There is currently a lack of effective and practical methods to reduce sleep inertia, but the study of exercise as a method of reducing sleep inertia may prove fruitful. AG
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