Health

Sleep lets the brain and body repair, consolidate memories, and clear waste. Humans spend about a third of their lives asleep.

Sunburn is inflammation caused by UV-B radiation damaging DNA in skin cells, triggering redness, swelling, and pain.

Wrinkles form because ageing skin produces less collagen and elastin, loses fat and moisture, and accumulates UV damage — the skin becomes thinner, drier, and less able to bounce back from repeated facial expressions.

Muscle cramps are sudden, involuntary contractions usually caused by muscle fatigue, dehydration, or imbalances in salts like sodium, potassium, and magnesium that disrupt normal nerve signals to the muscle.

Headaches arise from the activation of pain-sensitive structures around the brain — such as blood vessels, nerves, and muscles of the head and neck — since the brain tissue itself has no pain receptors.

Motion sickness happens when the balance sensors in your inner ear detect movement that your eyes do not — or vice versa — and the brain, confused by the mismatched signals, responds with nausea and dizziness.

A side stitch is a sharp pain below the ribs during exercise, most likely caused by irritation or stretching of the membrane that lines the abdomen, aggravated by heavy breathing and eating too soon before activity.

Most eyelid twitches, called myokymia, are harmless involuntary spasms of the orbicularis oculi muscle, usually triggered by stress, tiredness, caffeine or eye strain.

Ringing in the ears, known as tinnitus, usually happens when tiny hair cells in the inner ear are damaged or stressed, often by loud noise, and the brain fills the gap with a phantom sound.

Snoring happens when relaxed tissues in the throat and soft palate partly block the airway during sleep, making them vibrate as air flows past.

Pins and needles, or paraesthesia, happens when pressure on a nerve briefly disrupts its signals, and the tingling appears as the nerve recovers once pressure is released.

Pimples form when hair follicles become clogged with oil and dead skin cells, allowing bacteria to multiply and cause inflammation.

Jet lag happens when rapid travel across time zones leaves your internal body clock out of sync with local day and night.

Allergies occur when the immune system mistakes a harmless substance, such as pollen or peanuts, for a threat and overreacts by releasing chemicals like histamine.

Cavities form when mouth bacteria feed on sugars and produce acids that gradually dissolve the hard enamel of the teeth.

Hangovers come from a mix of dehydration, toxic acetaldehyde from breaking down alcohol, poor sleep, inflammation and chemicals called congeners in some drinks.

Dark circles appear mainly because the skin under the eyes is very thin, letting blood vessels show through, with genetics, ageing, allergies and tiredness making them more visible.

Earwax protects the ear canal by trapping dust, repelling water, keeping the skin moist and fighting bacteria, and it normally clears itself out.

Colds and flu spread more in winter because people crowd indoors, dry air helps viruses survive, and cold temperatures weaken some of the immune defences inside the nose.

Cold sores are caused by the herpes simplex virus, which hides in nerve cells and reactivates from time to time.

A popular theory blames falling air pressure before storms, but scientific evidence for a link is mixed.

Exercise boosts blood flow, releases growth factors and improves mood chemicals, supporting memory and mental health.

Vitamins are small nutrients the body cannot make in enough amounts but needs for vital chemical reactions.

Most adults naturally produce less lactase, the enzyme that digests milk sugar, after childhood.

The nose is lined with thin blood vessels close to the surface that can break easily from dry air, picking, blowing hard or minor bumps.

The skin under the eyes is very thin, and as we age or lose sleep, fat pads shift forward, fluid builds up and blood vessels show through, creating puffiness and dark shadows.

Leg cramps happen when a muscle suddenly contracts and won’t relax, often from dehydration, overuse, poor circulation or electrolyte imbalances.

Regular caffeine use causes the brain’s blood vessels to adapt; when caffeine is suddenly absent, vessels dilate and increased blood flow triggers a withdrawal headache.

Fever is the body’s deliberate response to infection: the immune system raises body temperature to slow down invading bacteria and viruses and speed up immune cell activity.

Eating quickly can distend the stomach and irritate the diaphragm or the phrenic nerve, triggering spasms that produce hiccups.

Lips have very thin skin with no oil glands, so they dry out faster than the rest of the face, especially in cold, dry or windy weather.

Tanning is the skin's defence response to UV damage: cells produce more melanin pigment to absorb future radiation.

Fingernails get more blood flow and more everyday use, which stimulates faster growth.

Alcohol suppresses vasopressin, the hormone that tells kidneys to save water, so more water leaves as urine.