THE BLOG

AM vs PM - When Should I Work Out?

29/09/2014 13:23 BST | Updated 29/11/2014 10:59 GMT

Warmly wrapped in a cocoon of bed sheets, the absolute last place most of us want to be is out in the gloomy morning drizzle, plodding along the pavement in Lycra shorts. Is a love of duvet cosiness likely to hamper your fitness levels, or are you justified in putting off your morning jog to a less painful time of day?

As you might expect, the best time to work out depends on what exactly you hope to get from your exercise. For those looking to achieve peak performance, sport science provides answers. The level at which our body performs fluctuates throughout the day, based on our circadian rhythm - the natural variation of internal conditions dictated by our body clock.

Strength, flexibility and short duration (i.e. non-endurance) performance have widely been found to peak in the late afternoon/early evening. Depending on the type of exercise, studies have reported an improvement of between 3 and 21% for afternoon performance compared with other times of day.

This peak in performance coincides with the circadian peak in body temperature - our core is warmest in the late afternoon. It is thought that this increased temperature acts as an internal warm up, helping our muscles perform at their best.

Hormone levels also vary with time and affect how well our bodies perform. Of particular importance is testosterone - it boosts protein synthesis so is key for muscle development. Testosterone levels are at a maximum early in the morning, but that doesn't mean that crunches at the breakfast table are a guaranteed shortcut to a toned physique. The hormonal benefits of morning exercise are offset by a similar early spike in cortisone, a hormone that acts in the opposite way to testosterone by breaking down protein.

But what if your goal is to shed pounds rather than smash records? There is a lot of conflicting opinion on the optimal time to burn through your excess body fat. A basic understanding of human metabolism suggests the early morning - after several hours of overnight fasting, blood sugar levels are low. This means a pre-breakfast workout will have to be fuelled by our body's excess energy stores - mainly fat. Research has confirmed that this "fasted state" exercise does burn through fat more quickly. However, there is also evidence to suggest that early morning exertion alters the body's metabolism for the rest of the day, making little overall difference to total fat loss.

With comprehensive studies thin on the ground, the jury is still out on whether this early morning exercise on an empty stomach really boosts fat fighting in the long term.

Rather than trying to take advantage of metabolic fluctuations, there is a far more straightforward way that most of us can find our optimum work out time. Regular exercise is better than occasional exertion, and even sporadic exercise is better than nothing at all. So before you rig up a home blood testing facility to analyse your personal testosterone peak, choose a workout schedule that fits with your daily routine, suits your personal preferences and is reasonable enough to become a habit.

For many people that does mean dragging themselves out of bed early, but if an evening jog is what is going to get your heart racing on a regular basis, don't feel guilty about pressing snooze.

Enjoy sweating!

References

Drust B et al. Circadian rythms in sports performance - an update. Chronobiology Int 2005; 22: 21-44

Chtourou H, Souissi N The effect of training at a specific time of day: a review. J Strength Cond Res 2012; 26: 1984-2005

Nicolas A et al. Effect of circadian rhythm of neuromuscular properties on muscle fatigue during concentric and eccentric isokinetic actions. Isokinet Exerc Sci 2007; 15: 117-29

Waterhouse J et al. The circadian rhythm of core temperature: origin and some implications for exercise performance. Chronobiology Int 2005; 22: 207-25.

Hayes L et al. Interactions of cortisol, testosterone, and resistance training: influence of circadian rhythms. Chronobiology Int 2010; 27: 675-705.

De Bock K et al. Exercise in the fasted state facilitates fibre type-specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans. J Physiol 2005; 564: 649-60.

Melanson EL et al. Exercise improves fat metabolism in muscle but does not increase 24-h fat oxidation. Exerc Sport Sci Rev 2009; 37: 93-101.