Why is bright light at night bad for sleep?

Bright light at night can suppress melatonin, increase alertness, delay your circadian clock, and disturb nighttime autonomic and metabolic rhythms—making it harder to fall asleep and stay asleep. 1

• 1. Short-wavelength blue light: Blue and other short-wavelength light are especially effective at suppressing melatonin through light-sensitive retinal pathways. But brightness matters too: sufficiently bright light of any color can affect the system. 2 3
• 2. Sleep timing and quality: Even small amounts of light late in the day can activate the circadian clock, make you feel like staying up later, and make it harder to fall asleep. Light during sleep can also raise heart rate, lower heart-rate variability, and increase next-morning insulin resistance. 4 5
• 3. Glucose regulation: In a study discussed on the show, one night sleeping in a room with moderate light was associated with increased next-morning insulin resistance, indicating worse glucose management. 6
• 4. Practical steps: Dim indoor lights after sunset, keep light low in the room, and avoid bright overhead lighting at night. If you need light, use a dim red or amber source; avoid turning on bright lights during nighttime awakenings when possible. 7 8