How can I simply and reliably track my computer usage?

I'm not after time tracking for the purpose of invoicing. I'm not even interested in which apps I use. I just want to know how much time I spend each day at my computer. One big pitfall I see is that many apps will suspend tracking when the user is watching a longer video or reading a long article. Are there any apps that work around this successfully, maybe by using the FaceTime camera as an additional source of data?