Mozilla got 4,000 volunteers to provide data about when they used the "private browsing" feature to surf the 'net without leaving a trace. The result: It's mostly at lunchtime (on office computers, I assume).
If I had a job that was so oppressive that I felt I couldn't browse the web for fun on my break without covering my tracks, I'd get out of the office at lunchtime.
Via FlowingData