So a lot of people make themselves work out early in the morning because it’s easier to find time then to work than it is to work out towards the end of the day. It’s less likely something will come along to derail your workout, whether circumstance or flagging motivation following a busy day.
I will note that in my long fitness history I’ve tried both working out very early and working out in the evening. I personally find there’s a lot I need to do to prepare for and get to work each morning, and I’m not usually clear headed enough to efficiently do most morning workouts either way. Others’ mileage will obviously vary.
The reason I don’t just set the alarm and wake up earlier is because the negative effect of losing sleep is greater than the positive effect of a morning workout, even if bio-rhythmically I come correct and learn to wake up earlier (and I already wake up naturally around 6am).
What happens if the previous night runs long or I otherwise have trouble getting to sleep? Now I spend the following day sleep deprived, along with all the negative hormonal effects of not getting enough sleep. The resulting cortisol and loss of growth/recovery hormones is actually a key behind lacking training results, faster aging, aging in general, not to mention illness and other psychological/health problems.
It’s more worth it to me to fit a workout in after work during the early evening, and it helps that I’ve developed the discipline to consistently do those workouts. Now and then I am able to get in a productive 6am workout after having slept well, but I realize that cannot be a daily thing with my current schedule and lifestyle… plus some workouts are too long for 6am to be a sufficient starting time.
So this leads me to talk about a couple things: