My laptop restarts during standby

Hello. I usually put my laptop into standby [lid down], which saves having to
reload documents and such. It's always powered.

The problem is, around 3 times a month I find that it's rebooted itself
during the night and is already on when I open the lid.

Anyone know what might be causing this and how to stop it?

p.s. I have a Dell Inspiron 1520.
Thank you in advance for your help.