If you only learn one thing about sleep, let it be the 90-Minute Rule.
Most of us were taught that “8 hours” is the magic number. But for a lot of people, 8 hours is actually a bad idea because it often forces you to wake up in the middle of a sleep cycle.
Think of a sleep cycle like a train journey. There are stations (light sleep), the middle of the tunnel (deep sleep), and the destination (REM sleep). If you try to jump off the train while it’s in the middle of a dark tunnel, you’re going to be disoriented.
By timing your wake-up call to land at the “station” (the end of a 90-minute cycle), you wake up feeling naturally alert. This is why some people feel better after 7.5 hours of sleep than they do after 8. It’s not about the quantity; it’s about the timing.