This really depends on what time they're used to going to bed!
People Who Go To Bed At A "Reasonable" Time
For somebody who is in a routine of going to bed before 11pm, waking up between 1am and 3am could be caused by:
- Middle-of-the-night insomnia (assuming that they have difficulty getting back to sleep). This is often brought on by stress or anxiety.
- Bedtime habits. Eating too soon before bed can disturb your sleep, and drinking too much before bed can cause you to wake up needing to use the bathroom.
- Not using up enough energy during the day. If you're not particularly tired when you go to bed, you might not sleep for very long - this often happens to me if I sleep in too late and then try to go to bed at a normal time.
- Stages of sleep. After a bout of REM (rapid eye movement) sleep - the stage during which you're most likely to remember your dreams - people often wake up. Even if they don't wake up completely, they enter into a very light sleep, which then makes them more easily disturbed by noises (dogs barking, cars outside, a partner's breathing, etc.)
People Who Are Used To Going To Bed Late
For people who aren't usually in bed until after 1am, it's perfectly normal for them to wake up if they attempt to go to sleep earlier.
Their bodies think they're just having a nap, and so several hours into sleep - which, for most people who stay up late, would put them at between 1am and 3am - they'll wake up, feeling ready to start the day.
This happens to me whenever I try to reset my sleeping pattern, and I usually get so frustrated with it, that I just give up.
The best thing I've found is to just humour your body and get out of bed. If I've tried to go to bed early, and it hasn't worked, I'll get up when I wake up (usually between 3am and 5am) and write or work for an hour or two, before going back to bed when I start to feel tired again. This probably doesn't help my sleep pattern in the long run, but it's preferable to lying in bed getting frustrated!