Friday January 30 2015
The study involved professional hockey players
"Our internal body clock has such a dramatic impact on sporting ability that it could alter the chances of Olympic gold," BBC News reports.
This headline comes from a study of 20 female athletes, which showed their peak performance on a fitness test was strongly linked to what are described as "circadian phenotypes".
These phenotypes were assessed using a questionnaire that looked at issues such as the time people tended to wake up and what times of the day they felt most active.
Depending on the results of the questionnaire, they were then classified into one of three groups: morning types (larks), intermediate types (let's call them "afternooners"), and evening types (owls).
They were then asked to take part in a fitness test known as the bleep test at different times of the day to see if there was a pattern in terms of peak performances.
And there was: the larks peaked around 12:00, the afternooners peaked around 16:00, and the owls peaked around 20:00.
Despite media reports to the contrary, this study wasn't saying anything about whether exercising at different times of the day is better for your health.
As an aside, there is a theory these results could explain the historical underachievement of the England football team.
Their body clocks have been set to play at 15:00 on a Saturday afternoon, but most World Cup games take place around 17:00 or 20:00. This is pure speculation at this stage, but good ammunition for post-match punditry.
Any form of exercise, whatever time of the day, brings important health benefits.
Where did the story come from?
The study was carried out by researchers from the University of Birmingham. No funding source was mentioned in the publication.
It was published in the peer-reviewed science journal, Current Biology.
The reporting on the BBC and Mail Online websites was generally accurate, and included a lot of comment from the study authors about the possible wider implications of their research, such as how Spanish footballers may have an advantage in the UEFA Championship League because they are more used to playing in the evening.
However, both of the news sources' headlines were misleading. The BBC said that, "Bedtime 'has huge impact on sport'," but the research wasn't concerned with when people went to bed: it focused on whether people were generally morning or evening types.
The Mail, on the other hand, said: "Morning jog? Leave it until noon" – but this advice only applies to larks, and only really if they are aiming to set a new personal best.
What kind of research was this?
This was an experimental study looking at how peak athletic performance is related to the time of day and people's individual circadian rhythms.
Circadian rhythms are biological cycles in the body related to the time of day. They are sometimes referred to as "the body clock", or as the body's "individual biological timing".
Historically, people have been categorised as "larks" or "owls". Larks – morning people – rise early, are most active in the morning, and feel awake shortly after they get up. However, they feel tired come late afternoon or early evening.
By contrast, owls – or evening types – don't feel fully awake until many hours after they get up. They remain somewhat tired during the morning hours, but become active and switched on in the evenings.
The researchers tell us circadian rhythms have been linked to athletic performance in past research, alongside many other factors.
They also tell us athletes appear to perform at their best in the evening. They wanted to explore whether this held true if you took account of whether people were larks or owls, or somewhere in the middle.
The study was small and designed to test a new hypothesis: a proof of concept study. It was not designed to provide definitive proof athletic performance is affected by the time of day, or is related to a person's biological clock. It was not large enough or diverse enough to achieve these aims.
What did the research involve?
The Birmingham study team recruited 20 competition-level female hockey players and asked them to perform their best at the bleep test.
This is a test of cardiovascular fitness involving a series of 20m runs in shorter and shorter times. Researchers performed the test at six different times of day between 07:00 and 22:00 to see how their performance varied.
Meanwhile, the women completed a new questionnaire specifically designed to study sleep/wake-related parameters, training, competition, and performance variables in athletes.
The team used the answers to categorise the women into:
- early circadian phenotype – "larks"
- late circadian phenotype – "owls"
- intermediate circadian phenotype – people more in the middle ("afternooners")
The analysis was pretty straightforward and appropriate.
What were the basic results?
Analysis by time of day, ignoring circadian phenotype
Overall, the results showed peak performance on the bleep test was in the late afternoon, around 16:00 and 19:00. Performance was lowest at 07:00. The variation between the best and worst performance throughout the day was 11.2%.
Analysis by time of day, taking circadian phenotype into account
When the team looked more closely at peak performance, they found it was significantly influenced by circadian phenotype. They found:
- larks peaked around 12:00
- intermediate types peaked around 16:00
- owls peaked around 20:00
The gap between the best and worst performance, when separated out by circadian phenotype, was 26% in the owls. It was less in larks (7.6% variation) and intermediates (10.0%)
To put this into context, the researchers reported the variation in time performance between first and seventh place at the 2012 London Olympic Games 100m sprint men's final was less than 5%.
They found peak performance was more related to the time people got up – specifically, the delay between that and competition – than the actual time of day.
Again, this varied a lot by circadian phenotype. Owls needed much longer after waking (around 11 hours) than larks before peak performance could be produced.
How did the researchers interpret the results?
The researchers stated the highlights of the research were findings that:
- athlete performance shows significant daytime variation
- personal best performance times differ significantly between circadian phenotypes
- internal biological time is the most reliable predictor of peak performance time
- daytime performance variations can be as pronounced as 26% in the course of a day
They concluded circadian rhythms, or internal biological time, are major determinants of athletic performance at different times of the day.
This study of 20 female athletes showed peak performance on a fitness test was linked to underlying biological timing, or what is called circadian phenotype. This was a better predictor of peak performance than the actual time of day.
The possible implications of the results were discussed widely in the media. Opinions ranged from a possible explanation for why Spanish teams do well in the Champions League (they must be full of evening types, which helps them perform best in the evening matches), to advice not to jog in the morning. A lot of this was speculative, so should be taken with a pinch of salt for now.
There is also potential confusion about what this means for people currently exercising and wanting to keep healthy. For clarity: this study didn't say exercising at different times of day is better for your health and fitness.
It says if you are competing, you may perform your best at different times of day, and this depends on whether you are more of a morning or an evening person.
Advice to ditch the morning jog until noon espoused in the Daily Mail doesn't really follow from this research, unless the aim of your morning jog is to break a personal best.
Similarly, this study isn't particularly relevant for people exercising to lose weight. It's more useful for athletes and coaches looking to optimise competitive performance.
The study authors advise internal biological time is more important than time of day, and we should listen to and understand the body clock more.
It would be interesting to know whether the circadian phenotype can be changed so, for example, athletes can prepare their bodies better for competition at a set time of day, even if this doesn't naturally fit with their lark/owl status. This study didn't address this question, but other related research might.
The study was small, only included women, and was designed mainly to show proof of concept. A larger, more diverse group (including men, for example) would need to be studied for us to be confident these results are applicable to the majority of athletes.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.