It turns out I’m far from alone. According to a 2023 meta-analysis that examined 91 studies across three continents, one in every five adults worldwide experienced general fatigue lasting up to six months, despite having no underlying medical conditions. In the US, 44% of the more than 1,000 adults surveyed by the National Sleep Foundation in 2019 said they felt sleepy between two to four days every week. While a 2022 YouGov poll of nearly 1,700 people found that one in eight UK adults were tired “all the time”, with another quarter knackered “most of the time”. Women were more likely to be fatigued than men, regardless of whether they had children or not — a finding that was echoed across multiple studies.

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But fatigue can also encompass a cognitive and emotional aspect – which explains why when we’re tired, we might experience brain fog, find it a slog to get things done, or snap at those around us. It’s only in the past decade that scientists have been able to dig deeper into these other facets of fatigue, thanks to advances in imaging technology and biochemical assays that allow us to study real-time changes in the brain, says Whittemore. “We’re really just beginning to understand the neurobiology now and what parts of the brain are perceiving fatigue.”

Another challenge is that fatigue is incredibly subjective, and it can arise for myriad reasons. It’s a symptom of many diseases and chronic conditions, including cancer, multiple sclerosis, long Covid, depression, and myalgic encephalomyelitis. However, it can also have much less serious causes. “It’s absolutely important to distinguish between illness and non-illness-related tiredness,” says Adam, who also teaches at the University of Aberdeen, where she is leading an ongoing study examining how fatigue impacts individuals with myeloma, heart failure, and long Covid.

The benefits of getting sufficient sleep – the amount that adults need varies, but most people need seven or more hours a night, and experts recommend getting between seven and nine hours – have been extolled upon us time and again. Without this downtime, our bodies wouldn’t be able to repair muscles, boost immunity, regulate emotions, consolidate memories and new information, among other critical functions. People who are fatigued for prolonged periods have a higher risk of death than the general population, as well as a higher risk of anxiety and depression.

On a daily basis, getting inadequate rest can result in headaches and other bodily pains, as well as cause feelings of irritability, low mood, and an unfocused mental state. These effects often spill over to our relationships. “We know from the literature on sleep and marital satisfaction that when one person in a marriage is sleep deprived, there’s more conflict in that couple,” says Barnes.

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But getting sufficient shuteye is only one part of the equation. Quality matters too, if not more. “Getting fewer hours of solid sleep is better than more hours of sleep, but interrupted sleep,” says Whittemore. “You feel much less refreshed if you have interrupted sleep.”

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The resting neurons create space for cerebrospinal fluid, which usually surrounds the brain, to flood in and clear out accumulated debris, such as the sticky beta-amyloid plaques that are usually associated with Alzheimer’s disease. It’s a sort of waste-clearance system scientists call the glymphatic system. “If you have disrupted or dysfunctional sleep, it disrupts that whole balance,” says Whittemore. “So there aren’t as many toxins being cleared from your brain.”

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This means that when we get our shuteye is important. Syncing sleep with our natural circadian rhythms – the brain’s 24-hour internal clock that regulates the cycle of alertness and sleepiness – gives rise to the best-quality rest. This explains why shift work is often associated with poor health outcomes, ranging from heartburn to diabetes.

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Dehydration is another major cause of fatigue. Other typical culprits include caffeine and alcohol. “I think most people underestimate how much they impact the quality of their sleep,” he says. “Caffeine, for instance, has a half-life of roughly five hours, which means even when you have a cup of coffee at noon, a quarter of that caffeine will remain at midnight.”