When 8 hours aren't enough
Even after 8 hours, you're still tired
Many people wake up feeling groggy even after getting a full 8 hours of sleep. If that’s ever happened to you, you're not alone—and you're not broken. The reason often lies not in how much you sleep, but when you sleep and wake up. To understand this, we need to talk about something called the circadian rhythm.
What is the circadian rythm?
Your circadian rhythm is your body’s internal clock. It runs on a roughly 24-hour cycle and controls when you feel alert and when you feel sleepy. It’s influenced by natural factors like light and darkness, and it affects everything from your energy levels to hormone production.
Think of it like a schedule your body tries to follow every day—telling your brain when to wake up, eat, rest, and sleep.
Not all sleep is equal
Sleep happens in cycles, each lasting about 90 minutes. These cycles have different stages: light sleep, deep sleep, and REM (rapid eye movement) sleep. You go through 4–6 of these cycles per night.
Source: Lieke WA. Hermans, Iris AM. Huijben , Hans van Gorp, Tim RM. Leufkens, Pedro Fonseca, Sebastiaan Overeem, Merel M. van Gilst. Representations of temporal sleep dynamics: Review and synthesis of the literature. Representations of temporal sleep dynamics: Review and synthesis of the literature - ScienceDirect
Waking up during deep sleep (which usually happens in the middle of a cycle) can make you feel groggy, slow, and disoriented. This is called sleep inertia. But if you wake up at the end of a cycle, when your brain is naturally more alert, you feel refreshed—even if you slept a bit less.
So if your alarm rings in the middle of a cycle, it can ruin the quality of your wake-up, no matter how many hours you've slept.
Why 8 Hours Isn’t a Magic Number
The idea that “8 hours is perfect” is an oversimplification. What matters more is whether your sleep aligns with your circadian rhythm and whether you wake up at the right stage of your sleep cycle. For example: If you sleep 9 hours but wake up in the middle of your deep sleep, you'll feel worse than if you sleep 6 hours and wake up at the end of a cycle.