Fatigue: Why do some people feel tired all the time?

Fatigue: For some people, no matter how much sleep they get, they still wake up exhausted. Scientists are still trying to understand why.

On most nights, the routine is impeccable. Pyjamas on before the first yawn appears. Teeth brushed. Skincare ritual completed with near-ceremonial dedication. The phone left outside the bedroom. Lights dimmed. The room cool but not cold — just right. A few lines scribbled into a gratitude journal. A chapter of fiction read under soft light. Sleep by 11pm.

Eight and a half hours later, the alarm rings.

And yet, the feeling is not refreshment — but fatigue.

For many people, this scenario is deeply familiar. They exercise. They eat reasonably well. They go to bed on time. They appear healthy. But they wake up feeling as if their battery never fully recharged.

They are far from alone.

A 2023 meta-analysis examining 91 studies across three continents found that roughly one in five adults worldwide reported prolonged fatigue lasting up to six months — despite having no diagnosed medical condition. In the United States, nearly half of adults surveyed by the National Sleep Foundation said they felt sleepy several days a week. In the UK, polling has found that one in eight adults feel tired “all the time,” with women reporting higher levels of fatigue than men.

Fatigue, it seems, has become one of modern life’s most common complaints.

A fuzzy, frustrating concept

Part of the challenge is that “tiredness” is not a single, clear-cut condition.

“It’s a very, very common complaint,” says Rosalind Adam, a GP in Scotland. The problem is so widespread that the UK’s National Health Service uses its own shorthand: TATT — Tired All The Time.

But fatigue is not the same as sleepiness.

Sleepiness is the tendency to drift off. Fatigue is broader. It can feel physical, like heavy limbs after a long hike. It can be cognitive, like brain fog and slowed thinking. It can be emotional, manifesting as irritability, low motivation or a sense of overwhelm.

Researchers are only recently beginning to untangle what is happening in the brain during fatigue. Advances in imaging and biochemical testing have allowed scientists to explore how the brain perceives and processes exhaustion. According to Vicky Whittemore of the US National Institutes of Health, the neurobiology of fatigue is still being mapped.

Fatigue is also deeply subjective. Two people can sleep the same number of hours and wake up feeling entirely different.

Quality over quantity

Most adults need seven to nine hours of sleep. But simply hitting that number is not enough.

Uninterrupted, high-quality sleep allows the brain to carry out crucial maintenance. During deep sleep, neurons shrink slightly, creating space for cerebrospinal fluid to wash through the brain and clear metabolic waste — including beta-amyloid proteins associated with Alzheimer’s disease. This cleaning network, known as the glymphatic system, works best when sleep is consistent and well-timed.

Interrupted sleep, however, disrupts that process. Even brief awakenings — from noise, stress, alcohol or breathing disturbances — can fragment deep sleep cycles. You may log eight hours in bed yet wake feeling unrefreshed.

“Getting fewer hours of solid sleep is better than more hours of interrupted sleep,” Whittemore explains.

When timing matters

Sleep is governed by the circadian rhythm — the body’s internal 24-hour clock.

According to sleep psychologist Daniel Jin Blum, sleeping outside your natural circadian window can reduce REM sleep — the stage linked to dreaming, emotional regulation and neural repair.

Shift workers often experience this disruption. Even if they sleep eight hours during the day, the misalignment with their internal clock can reduce restorative sleep stages. Over time, this misalignment has been associated with increased risks of metabolic and cardiovascular conditions.

In other words, when you sleep may matter as much as how long you sleep.

Stress: tired but wired

Chronic stress is another powerful contributor.

When the body perceives stress, it releases cortisol — a hormone that increases alertness and raises heart rate. In the short term, this is protective. But prolonged elevation can interfere with the ability to fall and stay asleep.

The result is a paradoxical state: exhausted yet restless.

Large population studies have shown that individuals experiencing significant stressful life events are far more likely to report persistent fatigue months later.

Hidden medical and lifestyle causes

When patients present with chronic fatigue, doctors first rule out medical explanations. Blood tests may reveal:

• Thyroid dysfunction
• Hormonal imbalances
• Iron deficiency
• Vitamin B12 or D deficiency
• Low magnesium levels

Nutrient deficiencies can impair energy production at the cellular level. However, in primary care, the majority of fatigue cases show normal laboratory results.

That shifts attention to lifestyle factors.

Dehydration, even mild, can reduce energy levels. Caffeine, with a half-life of around five hours, may linger in the system long after an afternoon cup of coffee. Alcohol can initially induce sleepiness but later disrupt REM sleep and increase awakenings.

Sleep disorders are another major cause. Snoring, often dismissed as harmless, may indicate obstructive sleep apnoea — a condition where breathing repeatedly stops and starts during the night. These micro-awakenings fragment deep sleep, leaving sufferers exhausted despite spending adequate time in bed.

The wider impact of fatigue

The effects of chronic tiredness extend beyond personal discomfort.

Research by Christopher Barnes at the University of Washington suggests that sleep-deprived leaders are more likely to display hostile or irritable behaviour at work. Fatigue has also been linked to increased workplace errors and accidents. In the UK, tiredness is estimated to contribute to roughly 20% of serious road accidents.

Fatigue does not stay contained within the individual. It ripples outward — into relationships, workplaces and communities.

So why are you still tired?

For otherwise healthy people, persistent fatigue often reflects a combination of factors:

• Fragmented or poor-quality sleep
• Circadian misalignment
• Chronic stress
• Undiagnosed sleep disorders
• Nutrient imbalances
• Emotional strain

It is rarely one single cause.

In many cases, the solution lies not in extending sleep time, but in improving sleep quality, reducing stress, adjusting timing, and addressing lifestyle habits.

The modern world often pushes people to optimise productivity, track sleep with devices, and perfect nightly routines. Yet fatigue persists.

Perhaps the real challenge is not simply sleeping more — but understanding how deeply interconnected sleep is with stress, biology, environment and daily behaviour.

For some, the answer may require medical guidance. For others, small adjustments may gradually restore energy.

But one thing is clear: feeling tired all the time is common — and science is only just beginning to understand why.

Also read: World Sleep Day: 7 Science-Backed Ways to Improve Your Sleep