Why do we sleep at night? Why night shifts are a risky business

I was inspired to write this post when I read about the story of Dr Lauren Connolly, in an article in the BMJ a few weeks ago.

Lauren, a 23-year-old from Glasgow, had recently qualified as a junior doctor. She was struggling to cope with hospital rotas which could mean working 14 hour night shifts, for more than 7 days in a row. On her final morning, after a night shift at Inverclyde Royal Hospital, she walked out of the hospital and into her car. She never made it home. Her car veered off the motorway at 9:40am, as she fell asleep at the wheel. Since Lauren's death in 2011, her father, Brian Connelly, has campaigned to raise awareness of the tragic consequences of relentless shift work and sleep neglect.

In this post I’ll start by explain the science underlying why working at night causes strain on the body, which means understanding how sleep is controlled.

Why do we normally sleep at night time?

We’re hard wired to sleep at night, every 24 hours. We have time-keeping built into the DNA of every cell in our bodies. You can see ups and downs every 24 hours in our core body temperature, heart rate, blood pressure, hormone production, reaction times and mood, to name a few. We’re programmed for rest and recovery when it’s dark at night, and for eating, optimum physical and mental activity when it’s light during the day. This internal rhythm is called the circadian rhythm, (circa=about, dian=day), also informally known as the body clock.

Diagram to show typical peak timings every 24 hours, produced by the Nobel Committee of Physiology or Medicine (Mattias Karlen)

Even if you switch all the lights off and live in a dark cave for a few months with no human contact, your internal sleep-wake rhythm will persist. (Folks really have tried this, but admittedly they succeeded decades ago, before social media addiction or Love Island were a thing.)

In fact, most of us have a rhythm which ticks over at a bit more than 24 hours, and the brain uses external nudges, called ‘Zeitgebers’ or ‘time-givers’ to keep us running in sync with our environment. (More on rhythms for night owls and early birds here.)

When we’re standing under a bright blue sky, directly exposed to the midday sun, we’re bathed in sunlight. Sunlight is particularly rich in blue or cyan wavelength light, which lands on the retina on the back of the eye and sends a powerful ‘Goooooo for it!’ alerting signal to a master body clock in the brain, which spreads the word to clocks throughout the body that it’s time for action.

Conversely, dim light after sunset is the natural cue for the brain to produce melatonin, the hormone which signals the body that you’re about to put your PJs on, and helps to prepare the body for sleep. In addition to light, food and physical activity are also Zeitgebers; eating and exercise send signals from muscles and from the gut to the brain telling it that it’s time to stay awake.

More recently, scientists have discovered that temperature is also a Zeitgeber influencing our internal clock. The body cooling is a prompt for sleep.. which helps to explain why sleeping in heatwaves is so tough.

Many of us intuitively know all this.. but I think it’s still a helpful to know why using light-emitting screens, late night curry or a 9pm HIIT class after 9pm lead to restless sleep.

So what happens when we stay up all night?

All the organs and processes that keep our bodies running smoothly operate on the expectation that every 24 hours, they’ll get a rest period when they have the chance to reset and repair themselves. So when we suddenly decide to mix things up, and stay up all night, we create a biological strain - our heart, immune defences, digestion, learning and memory, kidneys.. our cells can’t operate as efficiently, causing a risk of malfunction.

Typical circadian rhythm in heart rate, with a slowing during sleep overnight

To complicate matters, some of our internal clocks adjust faster than others, which can mean that not only are we out of sync with our environment, but some of our organs are out of sync with each other. We might be aware of some relatively minor consequences - headaches, feeling groggy or sick, but there are also serious hidden consequences to this ‘circadian misalignment’ beneath the skin that we we’re not consciously aware of…

In the case of metabolism for example, when the gut, liver, pancreas, muscle and fatty tissues are out of whack because we’re eating in the middle of the night, it can mean persistently high levels of glucose in the blood stream, which - over time - leads to an increased risk of diabetes.

It’s not that we can’t adjust to a change in routine - it’s just that it takes time.

In the same way that we eventually recover from jetlag, it takes about a day for the body’s internal clocks to recover from each hour that we shift our routine forward or back. Most changing shift patterns don’t allow the luxury of a gradual transition.

Why is it harder to sleep during the day?

Sleepiness is actually the product of two processes: the circadian rhythm, and another factor, called sleep pressure (also called your homeostatic sleep drive). The longer you’ve been awake, the more the pressure for sleep builds up. This is how we overcome the circadian alerting signal of daylight when we’ve been up all night - our sleep pressure becomes overwhelming, and we have to sleep.

Alertness is the product of the sleep drive and our circadian rhythm (Two process model of sleep, Borbely 1982)

Sleep pressure is due to an accumulation of a chemical called adenosine which builds up in the brain and makes you feel drowsy. Each time you sleep, the adenosine gradually disappears, and your sleep pressure resets.

If you wake up at 6am, and don’t nap during the day, by 10pm you’ve built up a significant sleep pressure AND (if you resist a social media overload) the darkness means your body clock tells you to sleep. In this case, both of your pro-sleep levers are aligned, and you fall asleep deeply.

We’re biologically adapted to sleep undisturbed when our body temperature is at its lowest. This naturally happens in the early hours of the morning, before dawn, and is unlikely to happen between night shifts during the day. Light during the day also stops the body producing melatonin.

So when we try and sleep during the day - even if we’ve built up a strong sleep pressure from having been on our feet all night - our internal body clocks are telling us to stay awake, so sleep is lighter and shorter; night shift workers typically have at least an hour less sleep overall than dayshift workers.

To compound the sleep challenge, from a practical standpoint, when you try and sleep during the day you have to contend with more ambient noise from traffic, family or co-habitants, deliveries, etc.

I'll follow up with a post on optimising sleep for shiftworkers. In the meantime..

You can find out more about Brian Connelly's campaign to change the working hours for junior doctors here.