Why we are grumpier after a bad night’s sleep remains a mystery as major theory is shot down

Scientists believed activity in certain brain regions were responsible for our bad mood after losing sleep until their study proved otherwise

It’s obvious a bad night sleep can make you grumpy – but the reason why remains a mystery to scientists.

Previously it’s been proposed that tired people react worse when faced with bad situations due to reduced brain activity.

But this theory has been shut down by Swedish scientists who, for the first time, scanned the brains of people who had been kept awake all night.

Scientists believed activity in certain brain regions were responsible for our bad mood after losing sleep until their study proved otherwise

The participants struggled to manage their emotions, according to the team of experts at Karolinska Institute.

However, to their surprise, the 80 volunteers didn’t show changes in certain regions of the brain as expected. 

The prefrontal cortex and amygdala are the decision-making and emotional areas of the brain respectively.

The academics, led by PhD student Sandra Tamm, suggested the two brain areas disconnect when someone is tired.

However, until this research – published in the journal Royal Society Open Science – the theory had never been tested. 

‘Sleep restriction has been proposed to cause impaired emotional processing and emotional regulation by inhibiting top-down control from prefrontal cortex to amygdala,’ the authors wrote.   

More than half of the sleep-deprived participants were aged 20–30, while the rest were between the ages of 65 and 75. 

After a normal night’s sleep, they performed a task to test how well they could hide their real emotions while looking at either negative or neutral images.

Participants were told to always look at the picture and MRI scans were recording their brain activity.  

They were then shown the pictures again without having brain scans, and had to rate how unpleasant they were on a seven point scale.   


Insomnia means you regularly have problems sleeping. It usually gets better by changing your sleeping habits.

You have insomnia if you regularly: find it hard to go to sleep, wake up several times during the night, lie awake at night, wake up early and can’t go back to sleep, still feel tired after waking up

Everyone needs different amounts of sleep. On average, adults need 7 to 9 hours, while children need 9 to 13 hours.

You probably don’t get enough sleep if you’re constantly tired during the day.

The most common causes of insomnia are: stress, anxiety or depression, excessive noise, an uncomfortable bed or alcohol, caffeine or nicotine.

Insomnia usually gets better by changing your sleeping habits. For example, going to bed and waking up at the same time every day, and only going to bed when you feel tired.

Source: NHS

The process was repeated again after the participants had been restricted to a sleep of three hours. It is not clear how they were kept awake. 

The study found that sleep restriction impaired the participant’s ability to control their emotions – something the participants reported themselves. 

Viewing negative pictures, irrespective of sleep condition and age group, triggered various activity in regions of the brain, including the amygdala.

But this had no significant meaning, the authors wrote in the journal Royal Society Open Science.

‘Sleep restriction caused a general decrease in rated success, but no effects on brain activity. 

‘At whole brain level, sleep restriction did not have any significant effect on the connectivity from amygdala to anywhere in the brain.

‘In conclusion, our data do not support the idea of a prefrontal-amygdala disconnect after sleep restriction.

‘And neural mechanisms underlying behavioural effects on emotional regulation after insufficient sleep require further investigation.’ 

In an attempt to explain why their theory had fallen through in comparison to other studies, the researchers said they may have actually given the participants too much sleep. 

‘Another possible cause may be that some of the subjects were partially sleep deprived in the full sleep condition and that the difference between the conditions was not enough to cause changes in the brain activity or connectivity,’ the authors said.

Evidence exists to show sleep loss can worsen anxiety and sadness, derail happiness and enthusiasm, make you more angry and eat unhealthily.

Sleep deprivation has even been shown to reduce DNA’s ability to repair itself, possibly leading to genetic diseases.

Regular poor sleep puts you at risk of serious medical conditions, including obesity, heart disease and diabetes, as the immune system is demolished.  

More than 40million people suffer from long-term sleep disorders in the US, data from the CDC shows. While it is estimated there are 1.5million patients in the UK.

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