Time course of sleep inertia after nighttime and daytime sleep episodes.

Self-ratings of alertness and reaction time were assessed after sleep, showing an increase in alertness and a decrease in reaction time within the first hour after awakening.

• Alertness improved and reaction time declined gradually after waking from both nighttime and daytime sleep episodes.
• The rate of increase in alertness was characterized by an exponential function with a time constant of 0.45 h.
• The rate of decrease in reaction time was also described by an exponential function, with a time constant of 0.3 h.
• These findings indicate that sleep inertia is a measurable phenomenon that affects cognitive performance immediately after waking.

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