Waking up is not an off/on switch, but a series of process in which different parts of the brain reactivate and reconnect. If these processes are not completed, it is possible to be awake but not fully functional – the state known as ‘sleep inertia’ - with transient confusion and performance impairment. Sleep inertia is often cited as an argument against workplace napping. These concerns prompted two studies investigating the duration and time course of sleep inertia after naps of different lengths.
Study 1 was designed to simulate napping during a night shift. The 12 participants came into the lab on four separate occasions. After a full night of sleep, they were kept awake for about 20 hours and then allowed naps of 20, 40 or 60 minutes ending at 2 am, or no nap. The extended time awake, and the timing of the nap heading into the sleepiest part of the circadian clock cycle, were expected to maximise sleep inertia.
Study 2 was designed to simulate extended operations. In this version of the protocol, after a full night of sleep participants were kept awake for 36 hours and then allowed naps of 20, 40 or 60 minutes ending at noon, or no nap. This combined the effects of a very long time awake with a part of the circadian clock cycle when alertness was expected to be increasing.
When they were woken from each nap (or at the equivalent time in the no-nap condition), participants performed a 6-minute test battery at frequent intervals. This included rating their subjective alertness and a working memory task. Their brain activity was also monitored.
In both protocols, no more than 15 min was required for performance decrements due to sleep inertia to dissipate after all naps. However subjective sleepiness was not a reliable indicator of this effect. With naps up to 60 min, there was no evidence that waking out of slow-wave sleep produced more severe sleep inertia. In study 2, after sleep inertia effects dissipated, the longer naps resulted in longer lasting improvements in performance.
It was concluded that naps of 40–60 min are recommended for sustained improvement in cognitive performance.
Funding: Study 1, United States Air Force, Asian Office of Aerospace Research and Development; Study 2, Marsden Fast Start grant awarded to Leigh Signal.