Sleep notice

[#1]

I keep getting a 'sleep' notice whether on the internet or on my files and the computer shuts down only to start up again. Sometimes this will happen again and again and sometimes I can get two or more hours uninterrupted use. Any ideas how to sort this?

[#2]

which operating system
I assume a variety of Windows so I have moved it to the correct board

[#3]

I'd check Scheduled Tasks and Power Options.