Sleeping in is one of the best parts of the weekend. After a long, exhausting work week, sometimes all you want to do is sleep. On Friday night, you slide under the covers, smiling in anticipation. Now you can finally catch up on your sleep.

Except you can’t. When you open your eyes Saturday morning, it’s still early. If this were a weekday, you’d be up before your alarm. So what’s the deal? 

We hate to break it to you, but you’ve pretty much done this to yourself. Your body is very good at recognizing patterns and adjusting accordingly. If you’ve got a 9-to-5 job, you’re getting up early five days a week. This effectively sets your body clock to wake you at a certain time each day.

Waking up may feel instantaneous, but it’s actually a pretty gradual process. About an hour before you wake, your blood pressure and body temperature rise, as do levels of stress hormones like cortisol. Minute by minute, you become more alert until you’re completely awake. You can shut off your alarm clock, but your body clock will just keep ticking.

There’s another reason that you probably don’t want to think about: You’re just not as young as you used to be. We need less sleep as we get older. Babies need between 16 and 20 hours. Teenagers should (but often don’t) get nine hours a night. Younger and middle-aged adults need about eight hours of sleep a night. In general, the older you get, the harder it is to snooze the day away. 

Even if you do manage to sleep in, you may never fully catch up on your sleep. If your body needs eight hours every night and you only get six or seven from Monday to Friday, you’d have to sleep an extra five to ten hours on Saturday to make it up. And experts say even that may not be enough

The bottom line: Sleep when you can, and enjoy those extra weekend morning hours. Who doesn’t want a longer weekend?