Definatley put the entire day in!!!Deer season(gun)here is only 4 weeks of the year,I can sleep the other 11 months.I'm not actually much of a stand sitter,rather I prefer a more aggressive approach to deer hunting in the form of stillhunting combined with rattling and vocalisations.I would say without a doubt that I have shot more deer during the "mid-day" hours,say from 10am-3pm,than I ever have at dawn or dusk.While the rest of the boys are in camp napping and eating,I'm out on the hardwood ridge gnawing on a hunk a bologna or chunk of cheese.I know one thing for fact,myself,nor anyone else I know,has EVER shot a deer from the camp bunk.
Another excellent reason to stay on stand is that most other hunters are early and late sitters.Generally,they leave their stands at 10ish and come back around 3pm?I see you are from S.Ont.From my limited experience hunting there for a few years during the 90's,I'll assume you are most likely sharing the woods with many others,yes?All that hunter movement at mid-day is bound to bump and push deer around,to the advantage of the hunter that remains on stand all day.
Finally,there are biological facts/reasons to support hunting all day.Deer,like other cud-chewing ungulates,tend to gorge themselves on food when they feel safe and then bed down in their safety zone to digest it.They have evolved this way out of neccessity,as they are most vulnerable to predation when they are pre-occupied with feeding.After an evening of nocturnal feeding,they make their way to bedding areas for a nap and some digesting.It's a fact that within 4-6 hours of feeding they generally get up to stretch,releive themselves,feed some more,and indeed do move about some during the mid-day.
If you only hunt 5 hours of the 10 hours of daylight available during deer season,IMHO you are missing out on 1/2 the deer season.