The thread on what do you listen to brought up this thought. I also don't recall this question ever being asked.
For me my most serious, concentrated modeling usually occurs in the late evening, as in 10:00 PM to 1:00 AM.
Earlier in the day I'm either working, shopping, home owner duties, honeydo's, watching TV/movies, etc.
The late evenings just offer less distractions. No phone calls, nothing on TV, stores are closed (well, not Walmart), to dark to work outside, everyone is in bed, etc.
I may apply a coat of paint or glue up some major parts before I start the above activities as it gives the project time to dry/cure for working later in the evening.