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At 3am, under a light you left on, your heart is beating faster than it should be. Your body never fully shifted into night. You dimmed the lights an hour before bed. You did the part everyone knows. This is about the light still on while you sleep. The charger LED, the cable box, the streetlight through the gap in the curtain.
Your eyelids are thin. They do not block light the way you assume. Enough passes through your closed eyes to reach the light sensors in your retina. Those sensors report to your brain that it is not fully night.
That signal keeps part of your nervous system in day mode. The branch that should be standing down stays partly switched on.
So your heart rate does not drop the way it should overnight. It should reach its lowest point while you sleep. Instead it runs high.
You wake unrested from a night that looked long enough. Rested on the clock. Wired in the body. This is the tired that has nothing to do with your supplements. You did the inside work. The room undid it.
In a study published in PNAS, healthy adults slept one night under dim light and one night under a moderate room light. The lit night raised their heart rate and reduced their heart rate variability while they slept.
Their eyes were closed the whole time. The light still reached them. It was a small study over a single night. The direction is what holds. Light while you sleep keeps your nervous system activated.
TONIGHT
In the last hour, walk the room and kill every light source before you get in. Cover or unplug the standby lights, and put tape over the ones you cannot switch off. Charge your phone in another room, face down is not enough. If a device must stay, turn its face to the wall.
If you air the room, block the light the open window lets in. Blackout curtains work, and a snug eye mask works when they are not enough.
The light you leave on in the last hour is the night your heart never gets to rest. Put the room in true dark, and your body finally drops into it.
