Thermoregulation and Sleep

Definition: The body’s process of maintaining a stable internal temperature despite changes in the external environment.
Importance: For humans, it ensures the body temperature stays around 37°C (98.6°F), which is crucial for optimal metabolic processes.
Mechanisms of Thermoregulation:
- Sweating: Evaporation of sweat cools the body.
- Shivering: Involuntary contraction of muscles generates heat.
- Altered Blood Flow: Blood flow to the skin is adjusted to manage heat loss or gain.
- Metabolic Rate Adjustments: The body can alter its metabolic rate to increase or decrease heat production.

Core Temperature: Approximately 37°C.
Homeotherms maintain body temperature within a narrow physiological range.
Balance Required: Thermoregulation involves a balance between heat gain and heat loss.
Implications of Imbalance:
- Hyperthermia: Occurs with excessive heat gain.
- Hypothermia: Occurs with excessive heat loss.
Heat Loss Mechanisms:
- Vasodilation: Blood vessels widen, increasing blood flow to the skin and promoting heat loss.
- Vasoconstriction: Blood vessels narrow to conserve heat.

Definition: The process by which the body generates heat, crucial for maintaining body temperature and supporting metabolic functions.
- Basal Metabolic Thermogenesis: Heat produced during normal metabolic processes at rest, such as:
- Cellular respiration
- Protein synthesis
- Ion transport
- This accounts for the majority of heat production in the body and is necessary for maintaining basic body functions.
Thermoeffector: Any physiological mechanism or process that regulates temperature by adjusting heat production, conservation, or loss.

Body Temperature Regulation:
- When body temperature falls:
- Blood vessels constrict to conserve heat.
- Sweat glands cease fluid secretion.
- Shivering generates heat to warm the body.
- When body temperature rises:
- Blood vessels dilate, promoting heat loss.
- Sweat glands secrete fluid to aid in cooling through evaporation.

Sympathetic activation results in:
- Arteriole narrowing, which reduces blood flow to skin capillaries, conserving core temperature.
- Decreased sympathetic tone causes arteriole widening, increasing blood flow to skin capillaries, thus enhancing heat loss and cooling the body.

Definition: The range of ambient temperatures within which the body can maintain its core temperature by regulating dry heat loss (skin blood flow).
Within the TNZ:
- The basal rate of heat production equals the rate of heat loss.
- Metabolic heat production changes with environmental temperatures.
TNZ Observations:
- To the left of TNZ (below LCT): Metabolic rate rises as the body activates non-shivering (NST) and then shivering thermogenesis (ST) to generate heat.
- To the right of TNZ (above UCT): Metabolic rate also increases for heat dissipation, through mechanisms such as sweating and vasodilation.

Radiation: Emission of electromagnetic waves transfers heat from the body to cooler surroundings.
Conduction: Direct heat transfer from one object to another.
Convection: Heat loss due to the movement of air or water around the body.
Evaporation: Conversion of water to vapor removes heat from the body into the air.

Preoptic Area (POA): The primary center for thermoregulation in the hypothalamus.
- Integrates thermal sensory information to control thermoeffector output (e.g., vasodilation, sweating, shivering).
Central Thermoreceptors: Located within the hypothalamus, these sense changes in blood temperature.
Peripheral Thermoreceptors: Found in the skin, they detect external temperature changes and communicate via the spinal cord.

Warm Sensitive Neurons:
- Comprise approximately 30% of the neurons in the POA and increase firing rate in response to rising local temperatures.
- Most are GABAergic and inhibit sympathetic drive to thermogenic effectors, reducing heat production.
Cold Sensitive Neurons:
- Make up less than 10% of the POA neurons, stimulating firing in response to decreased temperature.
Temperature-Insensitive Neurons: Comprise the remaining population of POA neurons.

Subjects: Adult mice exposed to different temperatures.
Methods: Warm challenge to induce thermoregulatory responses.
Measures: Neuroanatomical changes via pS6 immunolabeling and RNA sequencing in the VMPO of the hypothalamus.

Core body temperature follows a circadian rhythm that rises during the day and falls at night.
This rhythm:
- Promotes melatonin release.
- Increases sleep propensity, facilitating sleep onset and maintenance.
Disruptions (e.g., jet lag) can impair thermoregulation and sleep quality.

Body temperature naturally decreases during sleep, aligning with reduced metabolic rate and changes in thermoregulation.
The body’s temperature rhythm peaks in the afternoon and reaches its lowest point in the early morning.

Objective: Determine dependency of body temperature fluctuation rhythm on the circadian pacemaker (SCN) or sleep–wake cycle.
Methods: Continuous recording of body temperature and sleep-wake states using EEG/EMG.

Control Rats: Show a clear daily rhythm in temperature, which is lowest during consolidated sleep.
SCNx Rats: Lost circadian rhythmicity but still exhibited temperature drops during sleep.
Conclusion: Sleep-related drops in temperature are independent of circadian control.

Factor	Timescale	Effect on Core Temperature
Circadian Rhythm (SCN)	~24 hours	Sets overall daily rise and fall.
Sleep–Wake Cycle	Minutes to hours	Contributes to short-term fluctuations in core temperature.

Distal-Proximal Temperature Gradient (DPG): Temperature difference between distal (e.g., hands, feet) and proximal (e.g., torso, abdomen) body parts.
Physiological Function:
- A positive DPG indicates enhanced heat loss (peripheral vasodilation).
- A negative DPG indicates conserved heat (peripheral vasoconstriction).

Supports the role of distal vasodilation in initiating sleep:
- Greater distal vasodilation is associated with shorter sleep onset latency.
- Increased heat loss through extremities aids in falling asleep faster.

Definition: Episodes of excessive sweating during sleep, leading to soaked clothing or bedding, irrespective of room temperature.
Prevalence: Affects approximately 10–20% of the adult population.
Possible Causes Include:
- Hormonal fluctuations (e.g., menopause)
- Illness/infection (e.g., fever)
- Sleep disorders (e.g., obstructive sleep apnea)
- Medication side effects (e.g., antidepressants)
- Dietary factors (e.g., spicy foods, alcohol).

Thermoregulation is crucial for maintaining core body temperature.
Disruptions in either circadian rhythms or thermoregulatory functions can lead to impaired sleep and thermoregulation issues, exemplifying the intertwined nature of these physiological processes.