I was thinking about this question this morning as I dragged myself out of bed and got ready for a run. I was trying to get ready before I started to get hungry but also slow enough so that I could wake up a little more. I used to be such a morning workout person, I would wake up and go running and then do some weights. This habit always made breakfast a little more satisfying knowing that I had been so productive in the hours that lead up to it. However, since I started personal training in the early hours of the morning I have gotten out of this habit. There is no way I was going to get up, workout, eat and shower all before my 5:30 or 6:00am clients. Now, I have less AM clients and more work that needs to be done late mornings or early evenings. So, I have been trying to get back in the habit of waking up and working out right away but it has been very challenging getting back into this routine.
This all got me thinking back to my senior year research project, a topic I NEVER thought I would want to think about again. What time is the best time to work out? Is it early morning, mid-day, evening? This time last year I was spending hours upon hours in the library and in the exercise physiology lab researching this very topic. I was reading things on circadian rhythms, biological clocks, psychological responses to exercise and motivation levels at different times of the day and blah blah blah.
I also did a little experiment of my own where I had people run for 20 minutes, at a self selected speed at different times of the day (morning and evening). I recorded VO2 (oxygen consumption), RPE (how challenging the workout was), heart rate, RER (respiratory exchange ratio, basically compares the amount of carbon dioxide made and the amount of oxygen used by your body). Then I compared all those numbers using some fancy statistical program and I found that the only real difference was people ran faster in the afternoon when compared to the morning times.
My conclusions were basically that it depends on the person, if you are a morning person and you love working out in the morning then good for you! But, if you prefer to workout after work or after school then thats equally as good for you. UNLESS you are looking to perform better, (run faster in this case) then you are better off working out in the afternoon.
Of course, like any study mine had a number of limitations. The sample size was small, and people were only running, so the results may not be able to be generalized to all types of exercise. Also, all my subjects were college aged, and college students typically have different sleep schedules than other adults so there could be some error associated with that.
My take home message for this is that exercise is important for your health, and it is a necessary part of living a healthy life and taking care of your body. So fit in exercise whenever it works best for you and whenever you feel the best.