Muscles relax during sleep, including those in the face. The nervous system relaxes when we go into a deep sleep, which can also cause our facial muscles to relax. This can lead to our mouths falling open and drool escaping from the sides of our mouths. Also saliva can spill out of your mouth as drool when your facial muscles relax in your sleep like if the mouth falls open. Since the muscles around your mouth are relaxed, your mouth can be relaxed enough that saliva slips out. Once in sleep cycle, your body’s muscles, including those in your face and mouth, start to relax, often resulting in less swallowing and more drool. But sometimes when you're asleep, your brain forgets to tell your throat and mouth muscles to swallow, causing saliva to commute from your mouth to your pillow. As you sleep, your body enters a state of relaxation and restoration. This means that your muscles relax – including all of the muscles and tissues in the airway. When these tissues relax, they may fall back into the airway, partially blocking your ability to breathe normally. Sometimes causes your throat to compress as your tongue falls further back into your airway and the open space behind your tongue and soft palate is reduced. Inhaled air becomes turbulent. Directly inhaled air vibrates the soft tissues at the back of your mouth Though breathing is an involuntary function and it may be difficult to control how your breathe while sleeping, if you sleep on your side, the saliva collects in the side of your mouth and the reflex does not kick in to get rid of the drool. However, if you sleep on your back, saliva collects in the back of the throat and leads to automatic swallowing action. If you breathe through partially blocked nose, greater suction forces are created that can cause your throat to collapse and bring on snoring where your uvula and soft palate start to flap. When we fall asleep, many muscles in our body relax. This is true of the muscles in our airway, since not fully conscious. When you doze off and progress from a light sleep to a deep sleep, the muscles in the roof of your mouth (soft palate), tongue and/or throat relax more. This usually happens when the muscles in your body (including your face) relax during sleep, especially during your REM cycle. When this happens, your jaw falls slack and your mouth falls open.

NREM Stage N1 This stage of non-REM sleep is the typical transition from wakefulness to sleep and generally lasts only a few minutes. Stage N1 is the lightest stage of sleep; patients awakened from it usually don’t perceive that they were actually asleep During this stage: Eye movements are typically slow and rolling. heartbeat and breathing slow down muscles begin to relax you produce low amplitude mixed frequencies waves in the theta range (4 to 7 Hz) NREM Stage N2 This next stage of non-REM sleep comprises the largest percentage of total sleep time and is considered a lighter stage of sleep from which you can be awakened easily. This is the stage before you enter deep sleep. During this stage: heartbeat and breathing slow down further no eye movements body temperature drops Sleep spindles and K-complexes are two distinct brain wave features that appear for the first time NREM Stage N3 This final stage of non-REM sleep is the deepest sleep stage. Stage N3 sleep is known as slow-wave, or delta, sleep. Your body performs a variety of important health-promoting tasks in this final non-REM stage. During this stage: arousal from sleep is difficult heartbeat and breathing are at their slowest rate no eye movements body is fully relaxed delta brain waves are present tissue repair and growth, and cell regeneration occurs immune system strengthens REM Stage R There are two phases of REM sleep: phasic and tonic. Phasic REM sleep contains bursts of rapid eye movements, while tonic REM sleep does not. Stage R occurs about 90 minutes after you fall asleep, and is the primary “dreaming” stage of sleep. Stage R sleep lasts roughly 10 minutes the first time, increasing with each REM cycle. The final cycle of stage R may last roughly between 30 to 60 minutes. During this stage: eye movements become rapid during phasic REM breathing and heart rate increases and become more variable muscles become paralyzed, but twitches may occur brain activity is markedly increased When you fall asleep at night, you cycle through all of these stages of sleep multiple times — roughly every 90 minutes or so.