I've observed mule deer are definitely more active in the early hours from first light and that they are more likely to bed-down or at least move less after sometime around 9AM, especially if it's hot like anything near 40 deg. F. In the pre-light hours, I favor getting from camp to where I think the deer will be and moving into an advantageous position. It could be a bush or small tree that will provide some concealment or some rocky peak from which a large area can be glassed or just a ridgeline that can provide concealment while traversing. I've staked out water sources and ridgeline crossings before and had some very close encounters with does and fawns, but the buck encounters have all come from glassing and stalking and not still-hunting. As long as I can see the deer, I am either going to continue moving closer or waiting for the deer to move closer. If I am not seeing deer, I know I am less likely to see them in the late morning and by 11 AM I am probably going to be transitioning my activities. If I have a camp, I could go back to it to eat and rest, but I could also be moving my camp or resupplying my camp. I might have a camp that's a few miles from the truck and I could push it a few more miles beyond the wilderness boundary so that I can begin the hunt the next morning deeper in. If I've been out more than a few days, I might do a resupply run. This year, the camp was in weather down to -5 deg. F, so I needed a lot of fuel to melt snow or liquid water from the truck. I was 7 days out there, so that involved a lot of food and water.
The deer are more active again at some time after 4PM. The exact time depends on how late in the season it is, and whether it's daylight savings or standard time. This is a time that I'm likely to be looking for deer, looking for where they are and thinking about how to get there, but I am unlikely to take a shot. This year, the bucks were found at 9400 feet, thankfully in the morning. Imagine if one were shot at dusk at nearly 10,000 feet and had to be dressed and hauled out in subzero temperatures with the wind blasting on a moonless night while it was snowing in a wilderness where there are no trails, and that is if it could even be found after dark. I tried to be in camp by 5:30 (DST) because everything could be black by 6 and it was too risky.
So I'm hunting all day, but shooting is something that's done pretty much dawn to mid-morning only, not by rule but by probability and practical necessity.