It's easy to figure out what is happening here: early risers wake up with the alarm clock. This means they're probably running some sleep debt. I think late r

It's easy to figure out what is happening here: early risers wake up with the alarm clock. This means they're probably running some sleep debt. I think late risers listen to their body and get more sleep. This leads them to be more alert, because they're not running a deficit. They might need 1/2 hour more sleep, and get it (when needed).