| NREM sleep | REM sleep | |
| Autonomic nervous system | Increased parasympathetic tone | Unstable, brief surges in sympathetic and parasympathetic activity |
| Cardiovascular system | Heart rate is slow and regular Blood pressure is lower | Heart rate is irregular Transient increases in blood pressure |
| Respiratory system | Mild decrease in ventilatory drive in response to hypoxia and hypercapnia Decreased respiratory rate | Significant decrease in ventilatory drive in response to hypoxia and hypercapnia Increased respiratory rate |
| Temperature regulation | Decreased hypothalamic temperature set point | Reduced thermoregulatory mechanisms |
| Reproductive system | Penile erections Increased vaginal blood flow | |
| Dreaming | Less vivid | More vivid, visually intense |
