When nasal breathing doesn't work well, the body automatically switches to breathing through the mouth.
This is an emergency solution to get air in, not a resting state for the body.
Mouth breathing is shallower and less stable. As a result, oxygen is often absorbed less efficiently, especially during sleep. The body has to work harder, which keeps the nervous system more active.
That disrupts deep sleep.
You are more often briefly pulled out of deep sleep and miss important recovery phases without noticing. The brain is particularly sensitive to this, which can cause fatigue and a heavy feeling upon waking.
Snoring frequently occurs in this context.
Not as a cause in itself, but as a signal that the airway does not remain open at night. For this reason, persistent snoring is also seen as a possible early sign of more serious breathing problems during sleep, such as sleep apnea.
Not everyone who mouth breathes or snores has sleep apnea.
But it does indicate that nighttime breathing plays a significant role in how restorative your sleep is.