Why does lack of sleep make you grumpy?

Sleep tends to impact physical health and leaves the sleep-deprived person with headaches, lethargy and a general malaise. However, it does leave a sizable impact on their mental health as well. Sleepless nights tend to produce moodiness and a lack of good humour. But why? 

Why do some people get moody if they don't have enough sleep?

A research from the University of Arizona claims, the compactness of a type of brain tissue called white matter may influence a person's temperament when they don't get enough sleep.

Different people react differently to lack of sleep. Some are agitated and irritated, while some are nearly unaffected. 

Previous studies indicate that sleep is vital to cognitive, social, and physical function, can affect our attention span, and even increases anxiety while lowering our ability to manage stress and think positively. A study published earlier this year found that a genetic difference could allow some people to function on less sleep and the effects of a lack of sleep may not be easily reversible. But why does lack of sleep make some people easily irritable, or in other words, "grumpy"?

To determine why a person might seem irritable after enduring insufficient or disturbed sleep, researchers studied three networks of interaction within the brain: the default-mode (DMN), central executive (CEN), and salience networks. The

DMN is believed to be related to dreaming and may help us consolidate memories while asleep while the CEN is action-oriented. On the other hand, the salience network is believed to be responsible for integrating emotional and sensory stimuli and can mediate a switch between the DMN and CEN. Research suggests that interconnectivity between the three may impact psychiatric and neurological disorders.

At first, brains of 45 people were analysed using MRI scans as well as a specialized technique known as diffusion tensor imaging (DTI). A few days later the participants were tested again. They stayed awake overnight at the lab while they were asked questions based on their mood every hour between 7.15 pm and 11.15 in the following morning. In the meantime, the researchers looked at several regions within the brain. 

They found that participants who had greater white matter compactness, insulated fibres that connect the brain cells of grey matter together, were better able to stay positive even after sleep deprivation.

"We found that it is the compactness of white matter that is associated with greater resilience to mood degradation induced by 24 hours of sleep deprivation," stated study author Sahil Bajaj. "Very simply put, people with more compact white matter fibres seemed to be less likely to get 'moody' when sleep-deprived."

However, the researchers are quick to caution that much more research is needed to fully understand how a person's mood is impacted by a loss of sleep.