Morning or evening exercise: the best time is the one you keep.

Is it better to exercise in the morning or the evening? The alarm goes off at 6.15, it's dark outside, and you decide you'll go after work. At seven you leave the office with nothing left, and the gym is off again.

I'm asked this all the time. The short answer: the time you can keep to every week. Then you adapt the session.

Is it better to exercise in the morning or the evening?

If you can choose, late afternoon. For strength, that's usually when your body performs best. If your diary decides, both work.

• Start with last month. The time of the sessions you actually did is the first candidate.
• How you wake up. If you only get going mid-morning, you'll usually do better later on.
• Your sleep. If you're slow to drop off after an evening session, your time is earlier.
• Stress. In tense weeks, hard sessions sit better in the daytime.

What changes in your body from morning to evening

By late afternoon the engine is warm. Body temperature climbs through the day and peaks around five. Cortisol, the stress hormone, has dropped by then and, in men, testosterone has a small rise. That looks like the best time for heavy lifting.

In the morning you start cold. Body temperature is lowest between three and five at night, and at seven it's still low. In return, cortisol gives you a push, and after a session your attention stays sharper for at least two hours.

In the evening your body comes in to land. From about nine your temperature falls, and that fall helps sleep to come. A hard session raises adrenaline and temperature, so you may take longer to drop off. With age, evening cortisol tends to stay higher, which leaves less leeway.

How to adapt the session to your time

In the morning, warm up for longer. Walk briskly or cycle until you feel warm, then build up through your first sets. Going flat out straight away is like revving a cold engine to 3,000 revs.

Training fasted? Keep it light. After the night your blood sugar and insulin are low, so you use more fat during the effort. Afterwards, though, your body tends to use less. A walk is fine. Before weights, eat something small, like Greek yoghurt and fruit. Fasted, with cortisol high, your body may dip into muscle.

Evening exercise: leave three hours. Allow at least three hours between a hard session and bed, four if you tend to stay wired. If you're in bed by eleven, finish by half past seven, so a light dinner still sits three hours before bed. End with a few minutes' slow walking until your heart rate comes down. Skip it and you go home with your pulse racing and may struggle to drop off.

Then protect the slot. Same days, same time. With habit, the gap between morning and evening usually narrows. Exercising in the morning or the evening to lose weight? How many sessions you do in a month usually counts for more.

Open your diary and choose your two slots. Keep them for a month, then look at your weights and your sleep. They'll tell you whether you chose well.