There are two types of people in this world: There are those that are able to jump out of bed when the cock crows at 5.30am, throw on their trainers and get to a spin class before the sun's even up.
And then there are those that prefer to snooze the alarm, lay in bed and hit the weights room after dark.
But which is actually better for your body? Are those freakishly energetic gym larks doing it right, or should we be taking notes from the post-work tribe?
According to experts, there's no perfect answer. "The truth is there are benefits to both, and there are physiological differences between exercising in the morning or the evening," says Marvin Burton, head of fitness at Anytime Fitness UK .
Biologically, he explains, our individual circadian rhythms dip and rise at different times in the day, which means our energy, alertness and mental focus fluctuate.
Finding the time where your performance is at its peak for the type of exercise you like to do, is the key to sticking to a regular workout regimen.
"Studies have shown that the average individual who goes to bed at 11pm and has a 9-5 desk job displays the biggest spike in their blood pressure at 6.45am, as this is when the sleep hormone melatonin begins to drop," says Burton. When the body is exposed to natural daylight, the brain sends signals to produce the stress hormone cortisol too, which sets the body up for an early-morning energy boost.
For this reason, mornings have become a popular time for spinning and HIIT classes, but if you're not a natural early riser, Burton says activities such as steady state cardio walking, cycling or gentle forms of yoga, may actually be the best way to introduce your body to the day.
There are other benefits to getting in a workout first-thing too. "Training in the morning means you can benefit from a wakeup call, not only for you, but your metabolism too. Exercising in the morning ensures your metabolism gets going and it will burn [calories] more efficiently for the rest of the day," says Dave Mercer, a personal trainer at Nuffield Health (nuffieldhealth.com).
That said, sleep is a really important factor to any workout, and if you haven't had a sufficient night's kip, it might be better to snooze the alarm and slot in a workout later in the day.
"Morning workouts require getting a good night's sleep, a nutritious dinner the night before and a longer warm up to get your body ready for exercise," says Mercer. "Sleep is vital for the body to repair and recover, and you need to get enough so your body is in a good state to exercise the next day."
Don't discount the evening if you like to lift hard and heavy though. "If you're into strength training, the evening would biologically be the best time to do this," says Burton. "Exercising in the evening ensures you're alert, you'll have consumed enough calories and you'll likely feel like you have more energy.
"Be warned though, high-intensity training later in the evening, after 7pm, can negatively affect sleep - but a heavy lifting workout with plenty of rest and a lengthy cooldown period is a good alternative."
Bear in mind that you might also feel tired from your working day, so it's important to listen to what your body is telling you.
So, should you just do what feels right for you at the time then? "Yes. Unless you're an aspiring athlete or have the luxury of not having to work, my opinion would be to choose the one which allows you to plan and execute your daily tasks easily," says Burton.
"Personally, I'm a big advocate for morning workouts. Arguably it's a harder habit to implement, but you'll get a sense of fulfilment and accomplishment which sets you up for a proactive day.
"People who exercise in the morning often also benefit from feeling more alert, more productive and more energetic. The likelihood of something cropping up which would prevent you from exercising is lower too, so you're probably more likely to stick to a training plan."
The bottom line? There's no right answer. "Exercise is by no means a one-size-fits-all approach," says Burton, "and I always stress that it's personal for the individual."