*Geek Box: Time-of-Day Preference

*Geek Box: Chronotypes

All of our internal circadian rhythms are synchronised to the 24hr day, however, the exact timing of the peaks, troughs, and overall rhythm over that period may differ between individual. This is because individual responses to the environmental stimuli which entrain our biological rhythms may differ, giving rise to a spectrum of individual preferences for time of day.

Colloquially known as “larks” or “owls”, these respectively indicate a preference for mornings or evenings. Morning larks will naturally wake up earlier in the morning hours, and find it difficult to stay awake or concentrate late in the night. Conversely, night owls tend to have a preference to sleep later in the morning, and to be more nocturnal in their activity, with a preference for later sleep timing.

This may present difficulties because our social timing in society, from school start times to the traditional 9-5 workday, are at odds with the time-of-day preference of night owls, despite the fact that moderate to extreme night owls make up the majority of the population – it is the real morning larks that are rarer! There are also lifecycle differences in chronotype, and adolescents – long ridiculous for sleeping until 11am every day off from school – in fact need to sleep on the schedule, due to a natural delay in their chronotype that occurs during adolescents. This has important implications for cognitive performance in schools, and recent pilot studies have suggested that starting the school day later in the morning may increase academic performance.

For adults, particularly late chronotypes, the disconnect between social timing and desired sleep-wake timing may result in what is now termed ‘social jetlag’. There are also potential chronotype differences in metabolic health outcomes emerging, and night owls appear to be at increased risk for type-2 diabetes compared to morning larks, for reasons research is still attempting to elucidate. For research investigating the relationship between timing of food intake and health outcomes, chronotype is a critical consideration which may have a bearing on the results.