After arrival, why is a 20-minute nap usually safer than a long daytime sleep?

Answer: It protects night sleep

It protects night sleep ✓ — A short nap can reduce dangerous sleepiness without spending too much of the chance to sleep at local night. CDC guidance points to 20-30 minute daytime naps for alertness, and its traveler page suggests no more than 15-20 minutes. The trick is not that short naps cure jet lag; they buy wakefulness while preserving the next sleep anchor.

It completes adaptation — A nap does not complete circadian adaptation, especially after several time zones. The clock still has to respond to repeated light, dark, sleep, and activity cues. Thinking of a nap as "resetting" the trip misses the slower biological negotiation happening over the next few days.

It speeds clock shifting — Short naps do not directly speed the clock the way timed light or melatonin can. Long daytime sleep may actually compete with nighttime sleep and keep you exposed to the wrong cues. The practical surprise is that less daytime sleep can make the later sleep episode more useful.