How does light activate the suprachiasmatic nucleus?

Light helps tell our brain when it’s time to be awake or asleep by using a special part of the brain called the suprachiasmatic nucleus.

Imagine your brain has a tiny clock inside it, and that clock is right above where your eyes connect to your brain, like a little helper who listens to what's happening outside. This helper is the suprachiasmatic nucleus, or SCN for short.

When light comes in through your eyes, it sends messages all the way to the SCN. It’s like when you see sunlight coming through the window, that’s a signal telling your brain: “Hey, it's time to wake up!”

If there's no light, like at night, your brain gets the message: “Time for sleep!” The SCN uses these signals to control your body’s day and night cycle, which is also called the circadian rhythm.

So every morning when you open your eyes, it’s like the SCN is saying: “Good morning! Time to be active.” And every night when you close them, it's like it says: “Goodnight! Time for rest.”