Napping can help shift workers maintain alertness, but a nap that is too long can result in what's called 'sleep inertia'-the brief period of time of reduced alertness and impaired cognitive performance experienced immediately after waking from the nap. Minimizing sleep inertia may improve a worker's performance on the job. Some studies have shown that a 10 minute nap results in less sleep inertia than a 30 minute nap, possibly because the napper wakes from a lighter stage of sleep during the 10 minute nap. With a 30 minute nap, the napper is more likely to wake from slow wave sleep, which is the deepest level of sleep.
Relatively few studies on sleep inertia have been done where the night shift worker takes a nap during their work shift. This study examined the cognitive performance and subjective sleepiness immediately following either a 10-minute or 30-minute nap at 4 am. The researchers studied young, healthy volunteers in a research lab. Participants slept normally for the first night. The second night they were kept awake the entire night. One third were allowed no nap, another third were allowed the 10 minute nap, and the remaining third allowed the 30 minute nap. The participants did a battery of cognitive performance tests and subjective rating scales prior to the nap and 4 times during the first hour after the nap.
Remembering this is a small study, the results showed that 8/10 participants woke up in a lighter stage of sleep after a 10 minute nap. Also, 8/10 participants woke from slow wave sleep after the 30 minute nap. 3/10 participants entered into slow wave sleep in the 10 minute nap compared to 10/10 participants in the 30 minute nap. In other words, the longer nap allowed enough time for the participants to progress to the deepest level of sleep.
Regarding the post-nap testing, the 10 minute nap was associated with minimal sleep inertia and was helpful in slowing the performance decline seen in the group that took no nap at all. The 30 minute nap was associated with substantial sleep inertia. In addition, even though there was objective proof of reduced cognitive performance following the 30 minute nap, those participants rated their sleepiness as better after the longer nap. This means that the participant who napped 30 minutes overstated their ability to perform cognitive tasks during the hour following that nap. The study authors suggest that if night shift workers have to perform immediately post-nap, that the worker take only a 10 minute nap rather than a longer nap.