YouTube – How to Autorepeat a Video

www.timer-tab.com lets you do precisely that.

Look at the bottom, at settings, and paste a YouTube link in the "YouTube alarm" field.

You can use the t argument to make Timer Tab play the video at a predefined time. For example, if you enter the following address http://www.youtube.com/watch?v=YZX3lG306N4&t=10, Timer Tab will play the video at the 10th second.

Also, you can make Timer Tab repeat the YouTube video by appending &repeat to the YouTube link, e.g. http://www.youtube.com/watch?v=YZX3lG306N4&t=10&repeat. Which can be handy to make sure you wake up.

Disclosure: I wrote this web app.

Generally the limit is 15 minutes, although YouTube has raised this for users who have a good history of compliance with their rules.