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the fitness skeptic is a blog that takes a critical look at the health and fitness industry.

in it I'll examine the claims, products, practises and commonly held beliefs and SCRUTINISE the evidence.

My aim is to separate what is true from what is not and encourage fitness consumers and fitness professionals to become skeptics. 

I’ll take no prisoners when it comes to criticising the scam artist or highlightling the bogus but I’ll also give credit where credit is due.

Welcome to the fitness skeptic

Morning, noon or night; When is it best to exercise?

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Spend any significant time at your average health club or gym and you'll soon begin to notice how familiar the faces are at different times of the day. Ask the people exercising why they chose to be there at that time and you'd likely be told that they feel more alert and energetic in the morning or vice versa for those in the evening. This is all very well but what does the science have to say about it?  Does the time of day you exercise matter? 

Circadian rhythms

Circadian rhythms aren't the annoying sounds of the insects that keep your awake at night on holiday. They are in fact physiological and psychological changes in the body that follow a daily (24 hour) cycle.  They are found in most living things, including animals, plants and even microbial organisms. In humans (and other animals) the most influential external stimulus for synchronizing circadian rhythms to that 24 hour cycle is exposure to the light of day and darkness at night. 

However other external factors also influence our circadian rhythms such as our eating and drinking patterns, our exercise and perhaps most importantly the timings of our social interactions and work patterns. This perhaps goes some way to explain the differences between individuals and their preferences for which time of day they prefer to exercise.  

The exact effect these stimuli have on our bodies is complex and those without a degree in human biology need not worry themselves about the detail, suffice it to say they influence, among other things, blood pressure, body temperature, hormone levels and heart rate, all of which have a role in our body's readiness for exercise. 

Larks, Sparrows and Owls

Phenotype - the set of observable characteristics of an individual resulting from interaction with it's environment. 

Circadian rhythms do vary a little from person to person.  As I've already mentioned some people are morning people (let's call them Larks), some are evening people (let's call them Owls) and others, somewhere in between (Sparrows??).  Why this is the case seems to arise from specific genetic and physiological differences that impact how the individuals biological clock reacts to all the cues that produce the circadian rhythm and has been labeled our Circadian Phenotype. This all sounds very complicated (and it is) so unless you're about to embark on a PhD in chrono-physiology I wouldn't worry about the details.

The question is however, does all of this actually have an affect on our ability to exercise at different times of the day? 

Well, a recent (2015) study examining the effect of circadian rhythms on athletic performance attempted to find an answer. 

What they found was interesting and could have implications for the everyday and elite athlete alike.

Timing exercise to maximise performance

The study divided the participants into three groups using a chronometric test developed to identify circadian phenotypes. (Larks, Sparrows and Owls). They then all did an exercise test  at six different times of the day. 

On examining the results they found that the Larks performed best at about midday, Sparrows, around 4pm and Owls at about 8pm. They also noted that it was the Owls that suffered the most when it came to performing at different times of the day showing a 26% difference between their best and worst efforts compared to 7-10% difference in larks and sparrows. 

When factoring in time since awakening, Larks and Sparrows again showed little difference performing best about 5 hours (give or take) after waking up. Owls on the other hand didn't show optimum performance until a striking 11 or so hours after waking up! 

World records and World Cup woes

Ok, at this point it's worth remembering that the study was only really looking at peak performance and not the general benefits of exercising at different times of the day. None the less, the results are interesting on a number of levels. 

The first and most objective point is that your circadian phenotype is going to have an influence on what time of the day your are going to perform optimally. This may not be an issue to your average exerciser as breaking personal bests tends not to be a daily concern but to the elite performer it could have significant consequences. Championship events held  at different times of the day will potentially favour those with the appropriate body clock and those whose circadian rhythm is out of sync with the event time will be at a considerable disadvantage especially if you're a night owl trying to perform in the morning. A potential 7-26% drop in performance capability could be the difference between breaking the world record or finishing last!

It has been postulated that all of this may also explain why the England football team performs so badly at the World Cup! English footballers are brought up to peak at 3 in the afternoon when most top flight matches start in England, while most world cup matches start in the evening. Personally I think this is plucking at straws but I'm willing to suspend my disbelief for the sake of national pride. 

What about the rest of us?

Peak performance and elite athletes aside, what about ordinary people and everyday exercise? Could our body clocks and associated circadian phenotype impact the benefits we obtain? The simple answer to this is we don't know.

This study didn't address anything along those lines. It does however highlight the notable differences between larks, sparrows and owls which could, in terms of general exercise hint towards why different people find it so hard to exercise at different times of the day. If we suppose difficulty to exercise translates to quality of exercise, we could also speculate that a night owl would not be exercising optimally in the morning and vice versa for a lark. Further more, using the same logic we could hypothesise that morning exercisers may also be at a disadvantage when it comes to maximising the benefit they gain from exercise in their early morning sessions. Peak performance time for them seems to be about 5 hours after waking up, putting their 'best' exercise time at around midday and not in the morning. So perhaps a lunchtime session would be preferable? Owls on the other hand perform their best 11 hours after getting up placing their 'optimal' exercise time in a perfect slot for that post work visit to the gym. This is all of course speculation on my part. We'll have to wait until more research is done to see if any of it is true.

Body Clock Modification

Another potential area for future investigation could be on whether ones circadian phenotype could be modified. Could a lark become an owl? This could be particularly useful for athletes preparing themselves for competition at times of the day not suited to their current body clock. Or perhaps for someone wanting to exercise in the morning who currently has difficulty getting out of bed.

Final word

Based on our current understanding, it seems that circadian rhythms are important in terms of how our bodies are prepared for and how they respond to exercise. For athletes wanting to perform optimally, this may be something worth being concerned with. The rest of us however, should just be concerned with exercising when it's most convenient. Before work, at lunch time, in the evening or any combination thereof, what ever works best for you. Our body clocks will of course influence our preferences but unfortunately life sometimes gets in the way and we're stuck with little in the way of options. 

The bottom line is, if you're exercising on a regular basis, the time of day you do it will probably not detract significantly from the benefits you'll most definitely gain and that is of course the most important thing we should all be concerned about. 


The Impact of Circadian Phenotype and Time since Awakening on Diurnal Performance in Athletes. Current Biology, Feb 2015

Circadian Phenotype Composition is a Major Predictor of Diurnal Physical Performance in Teams. Frontiers in Neurology. Oct 2015 

Sleep, Biorhythms and Human Performance. Sports Medicine. Jan 1984

Circadian Rhythm and Human Health. Joan E. Roberts, Department of Natural Sciences, 
Fordham University July 2010.

Circadian Rhythms. National Institute of General Medical Sciences.  https://www.nigms.nih.gov/education/pages/Factsheet_CircadianRhythms.aspx










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