Why are you exhausted but your brain is fired up?

The clock reads 2.13am. You are exhausted. Your eyes ache, your body feels heavy and the alarm is already beginning to loom over the night, yet your brain refuses to let go. Instead, thoughts arrive in waves. Did you send that email? What if you forgot something important? Perhaps now is also the perfect time for your mind to replay a conversation from 2017 with forensic precision.

Many people recognise this frustrating state of being “wired but tired” – the paradoxical feeling of being physically exhausted but mentally unable to switch off. Surely tiredness should produce sleep automatically, but the brain does not simply fall asleep because the body is fatigued. In fact, under stress, exhaustion and sleeplessness often occur together. Part of the reason lies in the biology of survival.

The human stress response evolved to deal with immediate physical threats. For most of human history, danger tended to be extreme and short-lived – a predator nearby, an environmental hazard or conflict with another human group. In those moments, the brain’s priority was not rest but survival.

When the brain detects threat, a region called the amygdala initiates the body’s classic fight-or-flight response. Stress hormones including adrenaline and cortisol are released. Heart rate increases, breathing quickens...

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