Does sleep inertia affect cognitive performance?
Yes, significantly. Reaction time, memory and arithmetic are measurably impaired during sleep inertia; in one lab study, performance on waking was worse than after a whole night without sleep.
Documented impairments
Hilditch and McHill (2019) review lab and real-world studies of these deficits. Most of the impairment fades within 15 to 30 minutes after waking, but full recovery can take an hour or more, and the effect is larger after deep sleep: in one nap study, people woken from slow-wave sleep performed 41% worse than before the nap. Tasks involving attention, memory and arithmetic all suffer.
Comparable to a night without sleep
Wertz et al. (2006) found that performance on an addition test right after waking was more impaired than after a full night of sleep deprivation (as reported by Hilditch & McHill, 2019).
Practical implication
Avoid important decisions and driving in the first minutes after waking, when impairment is greatest. A wake-up mission is different: it is not about doing your best work, it simply keeps you up and moving while the grogginess fades.
Sources
- Hilditch & McHill, Nature and Science of Sleep, 2019. Sleep inertia: current insights
- Wertz et al., JAMA, 2006. Effects of sleep inertia on cognition
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