How long can you survive without sunlight?

Sunlight does one irreplaceable chemical job in the body: ultraviolet B converts a cholesterol derivative in skin into vitamin D. Everything else it does — setting the body clock, lifting mood — matters for wellbeing but is not lethal to lose. Submarine crews, polar winter station staff and long-stay cave researchers all go months without daylight and stay healthy on supplemented diets. Without either sun or dietary vitamin D, stored levels fall over a few months and calcium absorption drops, producing bone softening in adults and rickets in children. The circadian effect is real and arrives sooner: without light cues the sleep-wake cycle drifts, typically running slightly longer than 24 hours until it detaches from clock time entirely.

What changes the figure

• Oily fish, egg yolk and fortified foods supply vitamin D without any sun
• Bright artificial light at the right time of day holds the body clock steady
• Darker skin needs longer UV exposure to make the same amount of vitamin D

Physiology, not instruction. Every figure here is a population range under particular conditions, and individual variation is large — temperature, exertion, fitness and health move all of them substantially. Nothing on this page is medical advice.

Going without something

• How long can you survive without water? About three days in mild conditions — but under a day in desert heat, and up to a week at rest in the cold.
• How long can you survive without food? Weeks rather than days — one to three weeks is typical, and water runs out long before calories do.
• How long can you survive without air? Consciousness goes in about a minute and brain injury begins around four to six minutes.
• How long can you survive without sleep? No one has died of sleep deprivation alone; the record is 11 days, and severe impairment arrives within two.
• How long can you survive without food and water? About three days — water sets the limit and food barely enters into it.
• How long can you survive without human contact? Indefinitely in the direct sense — but isolation measurably raises the risk of dying early.
• How long can you survive without gravity? Indefinitely with exercise countermeasures — the record for one continuous flight is 437 days.
• How long can you survive without salt? Weeks to months at rest — kidneys are very good at conserving sodium — but days if you are sweating heavily.
• How long can you survive without vitamin C? Scurvy appears after roughly one to three months of near-zero intake, and is fatal in a few months more.
• How long can you survive without protein? Months, because the body cannibalises its own muscle — which is exactly why it eventually kills you.
• How long can someone with type 1 diabetes survive without insulin? Days. Missing insulin in type 1 diabetes is an emergency, not an inconvenience.