Light Is the Master Switch

Nothing tells your clock the time like light:

• Blue light from screens and bright bulbs reads as daytime and holds back the sleep hormone melatonin
• Morning sunlight is the strongest signal — about 10 to 15 minutes resets your clock for the day
• Soft evening light lets melatonin rise and gets your body ready for sleep
• Bright light at night confuses the system and can push sleep later by an hour or more

For most of human history, fire was the only light after dark. Now we stare into bright screens past midnight and wonder why sleep won't come.