Ask when you should work out and you will get two confident, contradictory answers. The evening people have the physiology on their side. The morning people have the calendar. Both are right, about different questions.
If the question is when your body is strongest, the research is fairly settled. A 2012 review by Hamdi Chtourou and Nizar Souissi in the Journal of Strength and Conditioning Research found that for anaerobic work, strength, power, sprints, performance follows a daily rhythm with its low in the early morning and its peak in the late afternoon or early evening [1]. Body temperature, hormone levels and joint stiffness all move through the day, and they favour the later session. For aerobic exercise the picture was less clear; some studies find an evening edge, some find none.
The same review adds a detail the evening camp tends to skip. Training regularly at a particular time of day shifts your rhythm towards it: the gains from training were greatest at the time of day people habitually trained [1]. If you always train at 7am, your 7am gets stronger. The morning penalty is real, and it is also partly a penalty for not being a morning trainer yet.
If the question is which workout you will still be doing in a year, the research points somewhere else entirely. Leah Schumacher and colleagues studied 375 physically active adults from the National Weight Control Registry, people who had lost significant weight and kept it off, and asked not when they exercised but whether they exercised at the same time [2].
Two thirds of them did, and the consistent exercisers did more: 4.8 days a week against 4.4, and 350 minutes a week against 285. The time of day itself did not matter for that effect; consistency at any hour was associated with more exercise. But of those with a consistent slot, nearly half had chosen early morning, between 4am and 9am. Not because morning is magic. Because it is the slot most likely to survive the day.
The best time to exercise is the one that is still there at the end of a bad week.
Habits form by repetition in a stable context. In Phillippa Lally's UCL study, the behaviours that became automatic were the ones performed after the same cue, day after day, and automaticity took an average of 66 days to arrive [3]. Wendy Wood's work at Duke found that around 43% of daily behaviour is already triggered this way, by context rather than by decision [4]. A workout that happens at the same time, after the same cue, is a workout on its way to running itself.
Evenings are a hostile environment for that. By 6pm the day has had its say: the meeting ran over, someone wants dinner, you are tired in a way that makes the sofa persuasive, and the cue for the workout was "when I get home", which turns out to mean different things on different days. The morning session has one enormous advantage: nothing has happened yet. The cue is the alarm, and the alarm is the one event in your day with a guaranteed time and a siren attached.
This is, not coincidentally, how First Light is built. The alarm is the cue. The mission is the smallest version of the workout you will do on your worst morning. The camera checks you are moving, the countdown runs only while you are, and the alarm does not stop until it is done. Nothing about that requires you to be a morning person. It only requires you to have set it the night before.
There is no evidence that ordinary exercise soon after waking is harmful. Strength and power are measurably lower first thing and joints are stiffer, so warm up for longer and expect the numbers to be smaller than an evening session. For a short habit-building session, that does not matter.
For a session of a few minutes, no. For anything long or hard, a small amount of carbohydrate helps. This is general guidance, not medical advice; if you have a condition that affects blood sugar, ask someone who knows your history.
Regular exercise at any time is associated with better sleep. Some people find vigorous exercise late in the evening delays their sleep, others do not. If evening sessions leave you wired, that is one more argument for the morning.
Then the honest advice is to pick a consistent afternoon or evening slot and protect it fiercely. The research says consistency matters more than the hour. Morning is just the easiest hour to keep consistent for most people, not the only one.