Sunlight makes you tired because heat widens blood vessels and lowers blood pressure, you lose fluids, and bright light affects sleep hormones.
Why
- Heat widens blood vessels and lowers blood pressure.
- Sweating loses fluids.
- Sunlight and heat raise body temperature.
- Bright light can change melatonin and circadian rhythms.
Prevention
- Drink water.
- Take breaks in shade.
- Wear sunscreen.
- Sleep well.
Heat illness
Heat exhaustion causes headache, nausea and heavy sweating.
Emergency
Confusion or fainting. Call 911.
Physiology of sun fatigue
Heat exposure dilates blood vessels, lowers blood pressure and increases sweating, which can cause dehydration and tiredness. The body works to maintain a core temperature of about 37 C (98.6 F). UV exposure triggers inflammation, and sunburn can cause fatigue and chills. Bright light also suppresses melatonin during day, but heat and glare from squinting add strain.
- Losing 1 to 2 percent of body weight in water impairs mood and concentration.
- Alcohol and caffeine increase dehydration.
- Heat exhaustion symptoms: heavy sweating, weakness, cold clammy skin, nausea, fast pulse. Treat by moving to shade, cooling and sipping water.
Prevention and emergencies
Drink water regularly, wear a hat and light clothing, use SPF 30 or higher, and take breaks in shade. Heat stroke occurs when body temperature exceeds 40 C (104 F) with confusion, hot dry or sweaty skin, and possible fainting. It is an emergency; call emergency services and cool the person with cold water or ice packs until help arrives. About 700 Americans die from heat-related illness each year according to the CDC, with higher counts in recent hot years.
Physiology of sun fatigue
Sunlight and heat make people tired for several reasons. Heat widens blood vessels, which lowers blood pressure and makes the heart work harder. Sweating causes loss of fluid and electrolytes, and even mild dehydration of about 1 to 2 percent of body weight can reduce alertness and mood. Bright light also affects melatonin, and the combination of warm temperature and long days can disturb sleep, leaving you tired the next day. Sunburn adds inflammation and fatigue.
Prevention and emergencies
Drink water, wear a hat and sunscreen with SPF 30 or higher, and rest in shade. Heat exhaustion causes heavy sweating, weakness, dizziness, nausea and a fast pulse, and needs cooling and fluids. Heat stroke, with body temperature above 40 C (104 F), confusion, hot dry skin or fainting, is an emergency: call 911. The CDC reports that heat related illness leads to thousands of emergency visits each summer in the US.
Heat, light and sleep chemistry
Exposure to bright sunlight suppresses melatonin during the day and resets the body clock, which improves night sleep, but strong heat plus UV can cause fatigue through dehydration and heat load. Sweat rates of 0.5 to 2 liters per hour in hot conditions are common. Losing 2 percent of body weight in fluid reduces mental performance. Sunburn triggers inflammatory chemicals called cytokines, causing tiredness in the first 24 hours.
Simple checks
Choose water or an oral rehydration solution, eat salty snacks if you sweat heavily, and seek shade between 10 a.m. and 4 p.m. when UV is highest. The UV index above 6 warrants sunscreen SPF 30 reapplied every 2 hours. Persistent exhaustion in the sun could also reflect anemia, low blood pressure, medications such as antihistamines and beta blockers, or a photosensitivity condition like lupus.
Based on data from the CDC and the American Academy of Dermatology
