Simply K

Your brain doesn’t care about “lights off” in the poetic sense — it cares about how much light actually hits your retina, specifically the melanopsin-containing cells in your eyes. Those cells feed directly into your suprachiasmatic nucleus, the brain’s body-clock boss, which tells your pineal gland whether to pump out melatonin (the “sleep” hormone) or not.

• Melanopsin : meh-luh-NOP-sin.
  
  It’s that pigment in your retinal ganglion cells that basically moonlights as a sleep-schedule manager — the one that freaks out the second you look at your phone at 1 a.m.
• Suprachiasmatric: soo-pruh-kai-az-MAT-ik.
  
  That little cluster of neurons sits just above your optic chiasm (hence the name) and tells your body, “Hey genius, it’s daytime,” or “Go to sleep already.”