How to best catch sleep and resume events for OSX?

I want to make a timer that counts whenever the system (Mac OS X Yosemite) is on. My idea is to use python and listen for sleep and resume signals and increment a counter whenever the system is on. However, I haven't been able to find much information on how to do this.

1. Would it be feasible to go about this problem with what I have detailed?
2. Is there a better way to solve this problem?

Thanks!