Why does sleep deprivation make us angry?

I almost cried when I heard S Club 7 on the radio...

True story: I had a really bad day, and was incredibly angry. When I heard their song on the radio I couldn’t help but get emotional. Don’t stop, never give up, hold your high and reach the top. It went on. When the world seems to get too tough, bring it all back to you. It was only once I had a good sleep later that night that I realised my day hadn’t been all that bad, and, worse yet, the words of S Club 7 weren’t that inspirational at all. I don’t even know what it means to “bring it all back to you.” It did make me think though, why am I so cranky when I don’t get enough sleep? And why was it all better once I had a good night rest?

Science has known for a while that disrupted sleep has a negative impact on our overall happiness, but we don’t know precisely why. Most studies talk about sleep difficulties and their link with psychiatric disorders like depression and bipolar disorder so it’s hard to interpret their results in terms of a run of the mill bad night sleep. Plus, whilst up to 90% of patients with depression are found to have sleep difficulties, we don’t know what came first, the sleep difficulties, or the disorder. There is also evidence that many alcoholics have insomnia, but again, we don’t know if people drink alcohol to get to sleep (meaning the sleep difficulties came first) or if people drink, and then find it harder to sleep. Whilst alcohol is a sedative, and will make you get to sleep faster, there is a rebound effect later in the night which leads to sleep disruption, and waking up in the middle of the night.

So what is happening when we sleep, and why would a lack of sleep lead to psychiatric disorders (including crying upon hearing seven Brits sing as song)? There are five of phases of sleep – appropriately called stage 1, 2, 3, 4 and rapid eye movement (REM). You usually begin at stage 1, move through each stage, and then go through the cycle again. Stage 1 sleep is a light sleep, where you might drift in and out of slumber. In stage 2 your eye movement stops, and your brain activity (which is defined by brain waves) becomes slower. Around 50 percent of your time sleeping is spent in stage 2. Stage 3 and 4 are the stages of deep sleep: your brain waves are a combination of slow waves and faster waves (moving into stage 4 there is almost exclusively slow waves). REM sleep is when dreaming occurs. It accounts for around 20 percent of sleep. When you enter REM your eyes move rapidly (as the name suggests), your muscles become immobile and your breathing becomes fast and irregular. Heart rate and blood pressure increase.

It’s thought that any crankiness that might be attributed to lack of sleep comes during Slow Wave Sleep (3 and 4). During a good Slow Wave Sleep, our body’s make less “fight and flight” hormones, like adrenaline and serotonin. So, our blood gets diverted from our brains to our stomach for some quality digestion, and our heart rate decreases. When we have a bad sleep, and wake up a lot, our bodies don’t do what they should – and instead more fight and flight hormones are produced. Higher levels of adrenaline and serotonin in our blood, means that instead of having a slow heart rate and slow, controlled breathing during slow wave sleep, our heart rate and blood pressure keeps pumping strong. Cortisol controls the release of “fight and flight” hormones throughout the body. It’s thought that during a bad night sleep, not enough cortisol is being made, so it can’t do its job properly. Studies have shown that patients who consistently make too little cortisol during the night have lower sleep continuity, and wake up more.

So why will this make me cranky in the morning? During the day, cortisol increases brain function and alertness. It also improves memory, and even suppresses of nonessential functions like eating. One theory says that when levels of cortisol being made at night are too low, it might lead to anti-social behaviour, lower alertness, and worse control of nonessential functions (which could definitely explain why I eat more when sleepy) the next day. But this theory is definitely in its infancy, as most of the evidence is related to patients who have chronically lower levels of cortisol, not just one bad night sleep. Aggressive adult males and incarcerated males who were habitual violent offenders have lower cortisol levels than “healthy controls”. Plus, where scientists created rats that were deficient in cortisol it led to antisocial behaviour (which if you’re curious, includes excess biting and not playing nice with the other rats).

Another idea is that serotonin, a hormone which usually decreases aggression, might change its function when there isn’t a whole lot of cortisol around. A study published in 2007 found that rats deficient in cortisol became more aggressive when they were given serotonin. So the combination of less cortisol, with more serotonin made during a bad sleep might equal one angry Wendy in the morning.

It’s hard to know exactly why we get cranky when we have a bad night sleep, but scientists look like they’re getting closer to an answer. In the meantime, I better stop listening to the top forty when I’m tired - if Hillary Duff came on the radio on that fateful day – who knows what could have happened?