'Night owls' vs 'larks': In battle of circadian rhythm, brain has final say

'Night owls' -- people who go to bed and get up later -- have fundamental differences in their brain function compared to 'morning larks', which may put constraints on their normal working day, a study has found. Researchers at the University of Birmingham in the UK and colleagues found that individuals whose internal body clock dictates that they go to bed and wake up very late have lower resting brain connectivity in many of the brain regions that are linked to the maintenance of consciousness.

This lower brain connectivity was associated with poorer attention, slower reactions and increased sleepiness throughout the hours of a typical working day, according to the study published in the journal SLEEP. It is already known that there are huge negative health consequences for night shift workers due to the constant disruption to sleep and body clocks, said researchers, including those from the University of Surrey in the UK and the University of Campinas in Brazil.

However, disruption can also be caused by being forced to fit into a societal 9-5 working day if those timings do not align with your natural biological rhythms, they said.

Since around 40-50 per cent of the population identify as having a preference for later bed times and for getting up after 8.20 am, the researchers say much more needs to be done to explore negative implications for this group. "A huge number of people struggle to deliver their best performance during work or school hours they are not naturally suited to," said Elise Facer-Childs from the University of Birmingham.

"There is a critical need to increase our understanding of these issues in order to minimise health risks in society, as well as maximise productivity," Facer-Childs said. Researchers investigated brain function at rest and linked it to the cognitive abilities of 38 individuals who were identified as either 'night owls' or 'morning larks' using physiological rhythms (melatonin and cortisol), continuous sleep/wake monitoring and questionnaires.

The volunteers underwent MRI scans, followed by a series of tasks, with testing sessions being undertaken at a range of different times during the day from 8am to 8pm. They were also asked to report on their levels of sleepiness.

Volunteers identified as morning larks reported to be least sleepy and had their fastest reaction time during the early morning tests, which was significantly better than night owls. However, night owls were least sleepy and had their fastest reaction time at 8pm in the evening, although this was not significantly better than the larks, highlighting that night owls are most disadvantaged in the morning.

The brain connectivity in the regions that could predict better performance and lower sleepiness was significantly higher in larks at all time points, suggesting that the resting state brain connectivity of night owls is impaired throughout the day (8am-8pm). "This mismatch between a person's biological time and social time -- which most of us have experienced in the form of jet lag -- is a common issue for night owls trying to follow a normal working day," said Facer-Childs, who is now based at the Monash Institute for Cognitive and Clinical Neurosciences in Australia.

"Our study is the first to show a potential intrinsic neuronal mechanism behind why 'night owls' may face cognitive disadvantages when being forced to fit into these constraints," Facer-Childs said.