I find this useful when tracing the time I spent in the computer or when I opened/closed my computer.
Find when you opened the lid:
$ tail -n 1000 /var/log/system.log | grep "Wake reason: EC.LidOpen (User)"
- Replace 1000 with the number of lines you wish to backtrack
- system.log might have already logrotated. gunzip if the past logs are already rotated: