I have that situation, but without the 'unwilling' part. In the office, I have often the situation "Gosh, I'd love to finish this, but if I don't catch the next train I have to wait another half hour for the next one", which automatically makes me stop work at fixed times.
Sure, I could also just stop at the same time at home, but ultimately I always end up working longer hours. Same in the morning; having to catch a train is really something that forces me to hit certain timings; when working from home, staying in bed just a little longer doesn't make me miss any trains.
It's a silly problem and could easily be overcome with enough willpower, but it requires the willpower where otherwise there are already external factors in place that soft-force me to start/stop work.