My wakeup time is determined by a wriggly 2 year old alarm clock that comes in to the bedroom at roughly the same time every morning (~7am) shouting "WAKE UP MUMMY AND DADDY! WHERE'S MY MILK?" (we're working on the pleasantries). Last night my sleep time was determined by same said wriggly 2 year old alarm clock who has a horrible cough/cold combo disrupting her sleep (and ours) throughout most of the night.
I have to leave the office at a certain time in order to pick her up from nursery, no chance of running late or getting someone else to do it. That hard cut-off to the end of my working day is very motivating as I know I don't have any buffer to stay a bit later to finish off something I'd said I'd do by the end of the day.
I got in to the office this morning and within 2 minutes I was programming away where I'd left off the night before. The commute in (cycling usually, train sometimes) is where I do the majority of thinking and planning for the day ahead.
There's very little email where I work and I keep my distractions in check with willpower and Leechblock (not got long left to post this).
I definitely believe that I'm more productive in these 8 hours a day (9 if you include commuting) than I was in the days where I'd be 'working' for 12-14 hours.
1. There's my hour a day exercise too.