The human brain serves as the control center for sleep, coordinating a system of neurons, hormones, and neural circuits that determine when we feel alert or drowsy. Within this three-pound (~ 1.5 kg) organ, tens of billions of neurons communicate through a vast network of electrical and chemical signals, while specialized regions like the brainstem and hypothalamus regulate our sleep-wake cycles. Neurotransmitters, the nervous system’s messengers, function as biological switches, with GABA and adenosine promoting sleep while norepinephrine and acetylcholine drive wakefulness. Hormones like melatonin synchronize sleep patterns across the body, operating alongside our internal biological clock. The nervous system extends this regulatory network throughout your body, influencing muscles, organs, and physiological processes during both sleep and wakefulness. All of these fundamental mechanisms explain why you feel drowsy at night and alert in the morning, providing the foundation for understanding the science of sleep and sleep disorders.