After a long flight, why is a brief daytime nap safer than sleeping for hours?

Answer: It protects nighttime sleep

It protects nighttime sleep ✓ — A short nap can reduce unsafe sleepiness without spending too much of the sleep pressure you need at local bedtime. It does not cure jet lag; it buys enough alertness to get through the day while keeping the next night as the main sleep anchor.

It finishes jet-lag recovery — A nap cannot finish jet-lag recovery by itself. Your body clock shifts through repeated cues such as light, darkness, meals, activity, and sleep timing, so adaptation usually takes several days after crossing time zones.

It repays all lost sleep — After travel, sleeping for hours in the daytime can feel like the fastest way to pay back a sleep debt. The tradeoff is that it may make local nighttime sleep harder, which can drag out the adjustment instead of helping it.