Last week's bad sleep is still affecting you right now

Sleep debt is cumulative. A few short nights don't reset when you finally sleep in on the weekend. The deficit carries forward, and your brain adapts to running on less in a way that makes it hard to notice how impaired you actually are.

One study put people on six hours a night for two weeks. By the end their cognitive performance matched someone who had been awake for 48 hours straight. None of them reported feeling that bad.

That's the part that makes it tricky. You stop feeling the deficit accurately after a while. You feel fine, or fine enough, but you're operating well below where you'd normally be.

One good night doesn't fix it either. Recovery from accumulated sleep debt takes days, sometimes weeks of consistent sleep before performance and mood actually return to baseline.