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Three Conditions
Falling asleep takes three things: the thoughts turn off, the heart rate drops, and the position of the limbs fades from awareness. The brain resists the first because so much of its function is predicting what comes next, so it plans, remembers, and organizes itself around the future.
Turning Thoughts Off
Turning thoughts off is a skill. The brain can also move into pure sensation, with attention traveling to the feet, the legs, the breathing, and the sounds in the room, then back again. That differs from meditation, which holds all attention on one place, such as the breath. It brings perception into the present, and the heart rate has to drop a certain amount before thoughts go quiet.
The Heart Slows
The heart gets that drop from respiratory sinus arrhythmia. The diaphragm moves and the lungs fill, so the space available to the heart changes with every breath. On the inhale the heart rate speeds up a bit. On the exhale the heart gets slightly smaller in volume, the blood in it moves quicker per unit volume, and the brain sends a signal through the vagus nerve to slow the heart down. When people are activated, they do a lot of inhaling and exhale passively.
Why does inhaling speed the heart up?
Breathing in lowers the pressure inside the chest, which pulls more blood back toward the heart. At the same time, the brain stem briefly reduces the vagus nerve's braking signal to the heart. The heart rate rises a little. On the exhale, the vagal signal returns and the rate falls. This rhythm is called respiratory sinus arrhythmia, and it is a normal sign of a healthy heart.
Why does the brain keep predicting the future?
A leading theory, called predictive processing, holds that the brain constantly generates expectations and compares them with incoming senses. Prediction saves effort because only the mismatches need to be processed. It also lets the brain act before events fully unfold, such as catching a ball. The same machinery is active when a person plans, replays memories, and imagines scenes.
How much does the heart rate drop during sleep?
Studies of overnight heart rate show it falling several beats per minute from evening wakefulness as sleep begins, and falling further in deep sleep, when it is lowest. In REM sleep, the rate becomes more variable and rises closer to waking levels. The drop reflects a shift toward parasympathetic activity, carried largely by the vagus nerve. Resting heart rate during sleep is often 10 to 20 percent lower than when awake.
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