What Happens When You Don’t Get Enough Sleep?

A look inside the body, not a scare story

There was a time when I thought feeling tired was just part of being good at my job. If you could still think clearly, still show up, still do what was required of you, then you were fine. Sleep was something you dealt with later, on days off, on holidays, when life slowed down.
Except it rarely did.

When people talk about not getting enough sleep, they often imagine heavy eyelids, slow reactions, or struggling to stay awake. But in real life, sleep loss rarely looks like that, at least not at first. It shows up as functioning. As coping. As getting through the day while something underneath feels slightly off.

Sleep is the body’s reset button - for hormones, the nervous system, and the brain. When sleep is shortened or fragmented, these processes don’t stop. They simply don’t finish.
You can still perform.
But the body is doing more work behind the scenes to make that possible.

So, what’s happening beneath the surface?
Sleep is the time when the immune system restores itself. During deep sleep, immune cells such as T-cells and natural killer cells are replenished; these are the cells that help the body respond to infections, manage inflammation, and identify abnormal cellular activity such as cancer growth. Research from UCLA has shown that even a single night of significantly reduced sleep can lead to a marked drop in natural killer cell activity by as much as 70 percent. These cells function as part of the body’s internal surveillance system, quietly scanning for potential threats. When sleep is shortened, that surveillance becomes less efficient. This tends to show up in everyday, unremarkable ways. People who average less than six hours of sleep are more likely to catch common infections, not because something is “wrong”, but because recovery has been incomplete. The immune system is still working,  just with fewer resources.

The brain is affected in a similar way. During sleep, particularly deep sleep, the brain activates a clearing system that removes metabolic waste accumulated throughout the day. This includes proteins such as beta-amyloid and tau, which are associated with neurodegenerative conditions like Alzheimer’s disease. Sleep is when the brain performs its most basic maintenance.
Learning and memory are closely tied to this process as well. Information taken in during the day is processed and stabilised at night. When sleep is reduced, concentration becomes less reliable, learning feels more effortful, and memory doesn’t settle as easily, even in people who are otherwise highly capable.

Sleep also influences how the body manages energy. With reduced sleep, insulin release becomes less effective, and cells respond less sensitively to it. Over time, this subtly shifts blood sugar regulation and makes weight maintenance more challenging, even when eating habits remain largely unchanged.
Appetite regulation changes alongside this. Signals of fullness become quieter, while signals of hunger become more pronounced. This often leads not only to increased appetite, but to a stronger pull toward quick, energy-dense foods. Cravings for refined carbohydrates are common in this state, reflecting the body’s attempt to compensate for limited recovery. This is not a failure of discipline or lack of motivation, but a physiological response to an under-recovered system attempting to meet ongoing demands.

Hormonal balance is particularly sensitive to sleep. Even a few consecutive nights of shortened sleep can lower key regulatory hormones such as testosterone and estrogen, subtly altering the internal environment the body is working within. These changes don’t always feel dramatic, but they do affect how resilient and balanced the system feels overall.
Alongside this, emotional regulation becomes more fragile, not in an obvious way, but in the sense that we become more reactive, less patient, and the ability to maintain a realistic perspective requires more conscious effort than it should.

Taking a breath of fresh air between the flights. Smile on, sleep postponed.

The body is remarkably adaptable, and many people build their professional lives on that adaptability, learning to function well under conditions that are far from ideal. But adaptation is not the same as recovery, and sleep debt is not something that can always be fully repaid at a later time, particularly when disruption is ongoing. The body compensates quietly, efficiently, and for longer than we expect, until that compensating state becomes the baseline.

Understanding sleep means moving beyond the idea of perfect routines or ideal conditions. It recognises that performance, decision-making, emotional regulation, and long-term health depend on biological processes that can’t simply be overridden by willpower alone.

Sleep isn’t something that fits neatly around the rest of life once everything else is under control; it is one of the systems that allows everything else to remain under control in the first place.

And sometimes, simply noticing that is enough to begin relating to rest differently.

Rest well.

Ivana