It could be a number of reasons:

- Too much sleep the night previous
- Working on something
- Having a job or class that requires you to stay up later
- Too much caffeine/sugar/other type of food or beverage
- Pain and/or itchiness and/or general uncomfortable sensation
- Certain medications can mess with your sleep schedule
- Certain mental disorders can also mess with your sleep schedule
- Other assorted reasons

And some of these can happen at the same time. 8Db