I usually look for most simple answer to problems like that (although, unless you are doing something all night with the system, such as downloading music or movies, it really isn't a problem so much as an inconvenience or annoyance).
The power supply that most modern systems contain will take the system down in the event of a power surge, spike or temporary drop in power. This could result in the system rebooting while having no effect on other appliances in your house, such as digital clocks, microwave ovens and VCR's. You would recognise this happening when you are awake as all of the lights in the house would suddenly dim then return to normal. If you find that this is indeed the problem, you may want to invest in a battery backup, also known as a UPS or Uninterruptable Power Supply (basically a surge protector on steroids that has a battery in it that can either keep the system running or, with the help of software that comes with it, shut the system down properly in the event of a power outage).
As a matter of prudence, I would suggest (to everyone running Windows (any version)) using the "Windows Update" feature to bring your system up to date with the latest security patches and software fixes. There have been several APM and hardware management patches released for most all Windows versions. It may not clear up the problem you are having, but it couldn't hurt.