The nervous system is responsible for controlling and coordinating the activities of the body. It is made up of the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system, which connects the central nervous system to the rest of the body. When the body is physically at rest, such as when sleeping or relaxed, different parts of the nervous system show varying levels of activity depending on their roles.
The Autonomic Nervous System
The part of the nervous system that is most active when the body is at rest is the autonomic nervous system (ANS). The ANS controls involuntary bodily functions such as heartbeat, breathing, digestion, and glandular activity. It works automatically without conscious effort and functions even when asleep or relaxed. The ANS is divided into two subsystems:
- Sympathetic nervous system – activates the “fight or flight” response
- Parasympathetic nervous system – activates the “rest and digest” response
When the body is at rest, the parasympathetic nervous system takes over to conserve energy and promote internal homeostasis. It slows the heart rate, increases intestinal and gland activity, and relaxes sphincter muscles in the gastrointestinal tract. The parasympathetic nerves responsible for many of theserestful functions originate from the brainstem and sacral spinal cord.
Key Functions of the Parasympathetic Nervous System at Rest
- Slows heart rate
- Constricts pupils
- Increases salivation and tear production
- Stimulates digestion and intestinal activity
- Relaxes bladder and rectal sphincters
The parasympathetic nervous system uses acetylcholine as its primary neurotransmitter to initiate these functions. Key parasympathetic nerves include the vagus nerve, which interfaces with the heart, lungs, and digestive tract, and the pelvic splanchnic nerves, which interface with the bladder and reproductive organs.
The Sympathetic Nervous System at Rest
While not as active as the parasympathetic system at rest, the sympathetic nervous system still maintains some important basal activity. It keeps blood vessels mildly constricted to maintain baseline blood pressure, and stimulates a small amount of glycogenolysis in the liver to maintain blood glucose levels.
However, overall sympathetic activity is greatly reduced compared to when awake and active. The major sympathetic nerves that originate from the thoracic and lumbar spinal cord regions fire at much lower rates to conserve energy.
The Brain at Rest
The brain also demonstrates changed activity patterns when the body is at physical rest. Energy consumption decreases, but certain regions remain actively engaged in functions like memory consolidation.
Decreased Brain Energy Use
Overall brain energy utilization drops during rest compared to wakeful activity. A resting brain consumes 20% less energy than an awake brain. Brain scans show that blood flow to cortical regions associated with wakeful cognition and sensory processing decreases significantly.
However, deeper brain structures in the limbic system and brainstem that regulate basic functions like cardiovascular activity, respiration, and thermoregulation maintain their activity levels.
The Role of Slow-Wave Sleep
Slow-wave NREM sleep is critically important for allowing the brain to rest and recharge. During slow-wave sleep, neurons synchronize their firing patterns resulting in slow oscillations of electrical activity. This synchronization facilitatesRestoration processes like protein synthesis, membrane trafficking, and synaptic homeostasis.
Slow-wave sleep is characterized by:
- Slower brain waves
- Reduced blood flow to cortical regions
- Decreased neuronal firing rates
Slow-wave sleep consumes less energy but allows the brain to be metabolically active in different ways than during wakefulness. The hypothalamus, which regulates sleep and circadian rhythms, remains engaged.
Memory Consolidation
Although cognitive capacity decreases during rest, important memory consolidation processes still take place involuntarily. The hippocampus replays recent memory sequences learned while awake in fast, compressed bursts.
This neural replay strengthens synaptic pathways between neurons to reinforce memory retention. Replaying learned information during rest freees up the hippocampus to better encode new memories when awake again.
Neurochemical Changes
The neurochemical environment of the brain also differs in a resting state compared to wakefulness. These changes promote restoration while reducing cognition.
Melatonin Secretion
The hormone melatonin is secreted by the pineal gland exclusively at night. Melatonin levels rise 2-3 hours before habitual bedtime, peaking in the middle of the night. It promotes drowsiness and works with the body’s circadian rhythms.
Reduced Norepinephrine
The neurotransmitter norepinephrine is associated with alertness, arousal, vigilance, and attention. Norepinephrine levels drop during rest to facilitate disengagement from the external environment.
Changes in Acetylcholine
Levels of the neurotransmitter acetylcholine fluctuate across sleep-wake cycles. It declines during slow-wave sleep but increases during REM to stimulate cortical activation and dreaming.
Increased Adenosine
The neuromodulator adenosine gradually accumulates in the brain during wakefulness as a byproduct of neuronal metabolism. It promotes sleepiness and inhibits wakefulness-promoting neurons. Adenosine levels decline during rest and sleep.
Other Changes in the Body at Rest
While the autonomic nervous system and brain show the biggest changes during rest, other bodily systems are also affected:
Lowered Core Temperature
Body temperature drops during rest phases, reaching its lowest point about 4 hours into a normal sleep period. This helps facilitate melatonin release.
Reduced Blood Pressure
Blood pressure decreases by 10-20% when resting compared to when awake. However, it fluctuates across sleep stages.
Decreased Metabolic Rate
Metabolic rate slows by around 15% during rest, lowering energy consumption. Brain glucose and oxygen uptake declines substantially during sleep.
Muscle Relaxation
Skeletal muscles enter a state of relaxation and increased blood flow after sustained contractile activity while awake. This allows metabolic wastes to be cleared.
Tissue Growth and Repair
Growth hormones are released during deep sleep to stimulate tissue repair and protein synthesis. The immune system also ramps up activity, reflected by increased cytokine production.
Changes in Heart Rate Variability
Heart rate variability, or fluctuations in the time between heartbeats, increases during rest. This reflects increased parasympathetic activity and cardiovascular health.
Circadian Influences on Resting Physiology
Many changes associated with rest follow a circadian rhythm aligned with nighttime sleep cycles. However, these changes still occur during daytime resting periods.
Afternoon Siestas
Afternoon siestas also involve decreased heart rate, brain glucose uptake, and slower brain waves compared to daytime wakefulness. Short naps may enhance alertness and performance.
Night Shift Alterations
People working overnight shifts show changes in blood pressure, body temperature, and melatonin release patterns that mirror their sleep/wake cycle rather than typical circadian timing. This may contribute to increased health risks.
Circadian Disruption
Maintaining proper circadian rhythms is important for cognitive performance and numerous metabolic and cellular processes. Disruption of circadian cycles through shift work or sleep disorders can negatively impact cardiovascular, metabolic, and mental health.
Key Factors That Influence Resting Physiology
Many factors affect the body’s resting state physiology:
Age
Deep NREM and REM sleep decrease with age, and the circadian rhythm advances earlier. Elderly people tend to wake up earlier and get sleepier earlier in the evening.
Drugs and Medications
Many prescription and recreational drugs alter neurotransmitters, brain activity, ANS function, and sleep architecture. These include adenosine blockers in coffee, acetylcholine inhibitors, serotonin modulators in antidepressants, and GABA agonists.
Medical Conditions
Diseases that affect the nervous, metabolic, endocrine, and cardiovascular systems often impair resting physiology. Examples include Parkinson’s, Alzheimer’s, hypothyroidism, and heart disease.
Mental Health
Mental health disorders like depression, anxiety, and schizophrenia frequently involve abnormal changes in ANS activity, sleep neurochemistry, and brain function at rest.
Lifestyle and Behaviors
Diet, exercise, substance use, technology exposure before bed, irregular sleep patterns, and chronic stress affect sleep quality, circadian rhythms, and ANS balance during rest.
Environment
Temperature, light exposure, noise levels, altitude, bedding comfort, and co-sleepers impact physiological functioning during rest phases.
Genetic Factors
Genes influence individual variation in circadian rhythms, sleep structure, and brain waves during rest through effects on neuronal signaling pathways, ion channels, and neurotransmitter systems.
The Resting Nervous System and Health
Adequate periods of rest, facilitated by appropriate functioning of the parasympathetic nervous system and changes in brain activity, are vital for sustaining physical and mental health.
Immune System Functioning
The parasympathetic nervous system and sleep cycles support immune system activity throughactions of the vagus nerve, increased cytokine production, and circadian regulation of whiteblood cells and proteins.
Metabolic Homeostasis
The ANS and sleep cycles help regulate glucose homeostasis, appetite signals, growth hormone release, temperature, and enzymatic activity involved in metabolism.
Cardiovascular Health
Parasympathetic dominance and blood pressure dipping at night are protective for the cardiovascular system. Disrupted circadian rhythms are linked to increased risk of heart disease.
Neuronal Restoration
Slow-wave sleep facilitates synaptic plasticity, memory consolidation, waste clearance, and other restorative processes in the brain that are impaired by lack of sleep.
Mental Health
Adequate parasympathetic activity and REM sleep are important for emotional regulation, processing memories and experiences, and preventing depression and anxiety.
Detoxification
The liver, kidneys, lungs, and other organs involved in cleansing the body of toxins and waste products work most effectively during rest periods.
Conclusion
The parasympathetic nervous system drives the body’s resting state physiology through actions that slow heart rate, increase intestinal activity, relax muscles, and conserve energy. The brain also shows changed activity patterns with key roles in memory consolidation. Other systems like the endocrine and cardiovascular systems demonstrate fluctuations that support restoration. Getting adequate rest facilitated by these nervous system changes is vital for physical and mental health.