Sleep is a restful state of reduced or absent consciousness that occurs in all mammals, all birds, and most reptiles, amphibians, and fish. The sleep cycle is divided into two broad categories: REM (rapid eye movement) and Non-REM (or NREM) sleep; NREM sleep is further divided into three stages. In Stage 1 sleep (N1), breathing slows and the brain’s electrical activity transitions from bursts to regular brains waves. Individuals in N1 are easily woken, and may not realize they’ve been asleep. This stage is sometimes accompanied by a “falling” sensation, that may cause a hypnic jerk or sleep start – an involuntary twitching of the muscles that might wake the sleeper.
After N1, a sleeper will enter Stage 2 sleep (N2). In N2, brain waves slow further, with occasional bursts of activity called sleep spindles. These spikes might reflect muscle twitching, activity in the cortex, or the brain integrating new information into what it already knows.
From N2, the sleeper descends into Stage 3 or N3 (formerly Stage 3 and Stage 4). N3 is also called slow-wave sleep (SWS) because the brain transmits very slow delta waves. As it’s the deepest level of sleep, individuals in N3 prove difficult to wake. During this stage, sleepers might suffer parasomnias: sleep disorders including somnambulism (sleepwalking), somniloquy (sleep-talking), bruxism (teeth grinding), and night terrors – periods of panic, fear, and confusion as the sleeper awakes.
Finally, there's REM sleep. As the acronym suggests, REM’s defining physiological features are rapid movement of the sleeper’s eyes, and paralysis of the body’s muscles called atonia. Although dreaming can occur in N2 and N3, REM dreams are most common and most vivid. REM lacks the slow, regular brain waves characteristic of other sleep stages; the brain displays erratic electrical activity, similar to wakefulness. On a typical night, a sleeper will experience N1 through N3 before entering REM for a few minutes; as the night progresses, these REM periods increase for a combined 60-90 minutes of REM dreaming.
Researchers disagree on REM dreaming’s purpose, but many think it allows the brain to reinforce and reorganize its countless synaptic pathways. This process forces random concepts, memories, and emotions into the sleeper's mind; to make sense of it all, the frontal lobe attempts to turn these stimuli into a coherent story: a dream. If one doesn’t sleep long enough to enter REM – or ingests alcohol or other REM-inhibiting substances – upon sleeping again, he will experience REM rebound and enter REM more quickly with more vivid and memorable dreams.
Although it’s still somewhat mysterious, sleep is known to repair body tissue, strengthen the immune system, and encourage learning and memory. Conversely, sleep debt or deficit can cause impaired judgment and memory, and increased risk of illness, depression and Type 2 Diabetes. Although it’s a hard lesson to live, perhaps we’d all be better off sleeping more and doing less.