How does sleep affect neural activity? To start, let's examine the typical adult sleep cycle.
When we go to sleep, we first experience "sleep-onset dreaming," or Stage 1 sleep, for 2-5 minutes. This stage is characterized by the gradual giving way of alpha waves, which are present during wakeful relaxation. Mental associations become looser as the body prepares for deeper sleep.
In Stage 2, heart rate slows and body temperature drops slightly. The thalamus, brainstem, prefrontal cortex, and cingulate cortex begin to decrease their activity in order to conserve energy. On an electroencephalogram (EEG), sleep spindles, or rapid spikes of neural activity, appear during this process. Because sleep spindles indicate neuronal firing, researchers hypothesize that Stage 2 sleep is vital for developing neural connections, measured as an increase in the synaptic strength between two neurons. Repeated high frequency stimulation of the presynaptic neuron results in the release of the neurotransmitter glutamate into the synaptic cleft. Glutamate binding over and over to a postsynaptic N-methyl D-aspartate (NMDA) receptor due to this repetitive stimulation can increase the strength of the synapse between the two neurons for hours, days, or a lifetime. Strengthening neural connections and solidifying synaptic pathways is at the core of learning and memory.
Stages 3 and 4 sleep compose slow-wave sleep. High amplitude, low frequency delta waves predominate brain activity. This type of brain wave differs greatly from the high-frequency waking alpha wave: delta waves originate from combined thalamus and reticular formation control. Researchers posit that much of sleep's mitigating effects on daily "wear and tear" occur during slow-wave sleep. Levels of cortisol, your body's stress hormone, drop nearly to zero, and levels of human growth hormone (HGH) rise. HGH promotes bone and muscle growth, stimulates protein synthesis, and helps to metabolize fats and carbohydrates. Slow-wave sleep is also thought to help eliminate unnecessary information accumulated during the day and store information deemed important.
Stage 5 sleep, or rapid eye movement (REM) sleep is characterized by beta waves. Increased neuronal activity produces beta waves, normally present in waking brain activity when individuals are active, working, or concentrating. Increased heart rate and increased blood delivery to the brain indicate higher levels of physiological activity. Eyes dart back and forth quickly under closed lids, and limb movements cease. REM sleep is where dreams occur, and where scientists think the link between sleep and long term memory consolidation lies: the hotly debated presence of theta waves during REM sleep may signal the conversion of recent memory into permanent storage of information.
One sleep cycle lasts approximately 90-100 minutes, and adults repeat the cycle about 4 times per nightly sleep session. Stage 1 sleep is bypassed in subsequent cycles. About half of sleep is spent in Stage 2, and about a fifth is spent in REM sleep.
It is crucial to remember that sleep is an active process, maintained by cholinergic cells. That is, sleep is governed by cells that use acetylcholine, an excitatory neurotransmitter, as a chemical signal.
It is crucial to remember that sleep is an active process, maintained by cholinergic cells. That is, sleep is governed by cells that use acetylcholine, an excitatory neurotransmitter, as a chemical signal.