A 90-minute nap is very different from a short power nap. It gives you time to move through multiple sleep stages, but it also removes more sleep pressure from the upcoming night.
Why people choose 90 minutes
The idea is that a longer nap may allow a more complete sleep cycle and reduce the chance of waking from deep sleep. Real sleep cycles vary, however, so 90 minutes is not a precise biological guarantee.
When a long nap may make sense
After major sleep loss, illness recovery or demanding travel, a longer daytime sleep may sometimes feel restorative. The trade-off is a greater chance of delaying nighttime sleep.
When to avoid it
If you already struggle with insomnia or tend to nap late, a 90-minute nap may make the next night harder. Shorter naps are easier to fit into a stable schedule.
Do not use naps to hide persistent sleepiness
If you regularly need long daytime naps despite adequate time in bed at night, consider discussing this with a healthcare professional.
Related: How long does a sleep cycle last?