What OS are you running?
If it's Win7 then the attached Event Log filter will display WakeUp events, (which includes Sleep/WakeUp times), for the last 30 days. Looking at the details for the events will also tell you whether the Hybernation file was written, (hyber.sys).
Open the Event Log and choose:
1) Import Custom View...
, select the Sleep-WakeUp.xml file and hit OK to get back to Event Log.
You'll now have a Custom View called:
, you can select the events and save as a CSV for import into Excel, etc. for a bit of data massaging. You just need the lines starting with Sleep Time and Wake Time, (the time is in UTC so you need to allow for timezone).
Event Source Kernel-Power
with an Event ID of 42
will also tell you when the system is entering Sleep mode but doesn't provide much info on why. I've only seen two reasons on my PC: Application API or System Idle.
There's also the free Event Log Explorer
, (register for the free license), that'll work on WinNT and later.