The science of sleep stages - explained simply

We all know sleep is important. But have you ever wondered what is actually happening in your body and brain while you’re asleep?

Sleep isn’t just one big block of “rest.” It’s a cycle of stages, each with its own role in keeping you healthy, happy, and functioning at your best.

Here I’ll break it down in a really simple way, so you can see why every stage of sleep matters.

The cycles.

You don’t just go through these stages once, you move through them in around 90-120 minute cycles (not an exact science), these repeat 4–6 times each night. Early in the night, deep sleep dominates. Later in the night, REM stages get longer and richer. That means if your sleep is cut short, you’re most likely to miss out on that vital REM-heavy second half of the night.

Why it matters.

When people say “quality over quantity,” this is exactly what they mean. Seven hours of sleep with good cycling through all stages can be far more restorative than nine hours of broken, disrupted sleep. And remember: if your sleep has been rocky, your body is clever. It will prioritise catching up on the stages you need most. You won’t stay “broken” forever.

Key takeaway.

Every stage of sleep has its job, light sleep helps you settle, deep sleep restores your body, and REM sharpens your mind. Together, they’re your natural overnight toolkit for health, energy, and resilience. And if sleep isn’t coming easily, it’s often the REM stage that’s most affected, explaining why you feel the effects of sleep loss so much in your mood and focus.