Stage	Sleep	Breathing
Stage 1	light sleep	Unstable
Stage 2	stable sleep	Very regular in A and F
Stage 3	restorative sleep	Very regular in A and F, reduced UAD (upper airway dilator) muscle activity
Stage 4 (REM)	dream sleep	Erratic, shallow breathing with irregular F and A, loss of intercostal muscle activity, most reduced UAD muscle activity