How do naps affect nighttime sleep quality?

Short naps of 10 to 20 minutes generally have little negative effect on nighttime sleep and can improve alertness and mood. However, longer naps exceeding 30 minutes can cause sleep inertia and reduce sleep pressure, which is the biological drive to sleep that builds throughout the day. This reduction in sleep pressure can make it harder to fall asleep at night and may fragment nighttime sleep. Timing matters significantly. Napping after 3 pm is more likely to interfere with nighttime sleep because it occurs too close to bedtime. People with insomnia are typically advised to avoid naps altogether, as they need to build maximum sleep pressure by bedtime. For healthy sleepers, an early afternoon nap kept under 20 minutes is usually safe.