Biological & Circadian Rhythms

• Biological rhythms are patterns, rhythms, and cycles, including circadian (24-hour, sleep-wake cycles) and seasonal patterns (cycles over months).

Circadian Rhythms

• Blood pressure usually peaks in the morning, which correlates with a higher incidence of heart attacks during this time.
• Melatonin secretion decreases in the morning, reducing sleepiness. Shifts in circadian rhythm can alter melatonin secretion.
• Cortisol, a stress hormone, peaks in the morning to promote wakefulness; individuals with insomnia may experience elevated cortisol levels before sleep.
• Jean-Jacques d’Ortous de Mairon (1729) found that mimosa plants continue to follow opening and closing rhythms even in complete darkness, demonstrating an internal clock regulates biological rhythms.
• Nathaniel Kleitman spent 32 days in a cave to isolate from external cues, discovering body temperature follows a 24-hour cycle and documenting circadian rhythms in humans.

Behavioral Rhythms in Animals

• Actograms measure sleep cycles based on movement, showing activity patterns (e.g., hamsters active at night).

Molecular Basis of Circadian Rhythms

• Jeffrey Hall, Michael Rosbash, and Michael Young (2017 Nobel Prize) identified genes in fruit flies that regulate circadian rhythms, revealing a molecular clock mechanism with proteins (e.g., PER proteins) in a self-regulating process.
• Michael Rosbash stated the circadian system anticipates cycles, considering light/dark patterns.

Discovery of Sleep EEG

• Hans Berger (1924) invented the EEG and (1931) discovered alpha waves (8-13 Hz) slow down during sleep, but his findings were not recognized until 1937.
• Nathaniel Kleitman built the first sleep laboratory where he and student Eugene Aserinsky (1953) discovered rapid eye movement (REM) sleep, which is associated with vivid dreams and brain activity similar to wakefulness.

Sleep EEG/Polysomnography (PSG)

• EOG (Electrooculogram) measures eye movements.
• EMG (Electromyogram) measures muscle tone.
• EEG (Electroencephalogram) measures brain activity.

Sleep Stages

• Alpha waves indicate an awake state.
• N1, N2, and N3 are sleep stages 1, 2, and 3; N3 is a restorative stage with large, slow waves (slow-wave sleep).
• Each 30-second recording epoch is visually scored.

Main Sleep Variables

• Lights on and off times.
• Time in bed (between lights off and lights on).
• Sleep onset latency (time to fall asleep).
• REM latency (time to reach first REM state).
• Wake after sleep onset.
• Total sleep time.
• Sleep efficiency.
• Absolute and relative sleep stages.

Functions of Sleep (Theories)

Survival (Biologically Adaptive)

• Optimal energy regulation involves searching for food during optimal periods and conserving energy via sleep at other times; animals with nutrient-rich diets sleep more.
• Sleep is a state of fasting/physical inactivity that slows metabolism (except during REM), conserving energy.
• Safety is increased by sleeping when it is unsafe to move, and prey animals have more frequent, short periods of REM sleep for quick alertness, increasing the chance of survival.

Emotional Maturation

• Babies begin to smile during sleep, especially during REM sleep, before smiling while awake. Which suggests a connection between emotional development and REM sleep.

Emotional Regulation

• Emotional processing involves changes in emotional evaluations after taking a nap, particularly with REM sleep, and facilitates habituation to emotional stimuli, as indicated by reduced skin conductance and muscle tone responses.
• Acute sleep deprivation increases amygdala activation in response to negative stimuli and pleasure-evoking stimuli, heightening emotional lability; it can have a transient antidepressant effect, though prolonged deprivation worsens depression.

Physiological Restoration

• Increased daytime brain activity and physical activity lead to increased SWS (N3) in the subsequent sleep episode.
• Growth hormone is released during SWS, aiding physiological development and tissue repair/regeneration.
• Immune functions are promoted, as infections increase SWS. Sleep loss reduces antibody production after vaccination and decreases cancer-fighting immune cells; short sleep increases the risk for certain cancers, leading the WHO to classify shift work as a carcinogen.
• Brain cleaning occurs during sleep via expansion of extracellular space and increased CSF circulation, removing neurotoxins.
• Better cardiovascular regulation is enabled through decreases in heart rate and blood pressure during sleep, rebooting the system. Poor sleepers have a higher risk of coronary artery calcification, heart attack, and stroke.

Restoration of Endocrine Functions

• Reproductive hormones are affected as men sleeping less have lower testosterone levels.
• Regulation of appetite/eating behavior: Partial sleep loss increases ghrelin and decreases leptin, increasing hunger for high-carbohydrate foods.
• Restoration of metabolic functions involves stable glucose levels and slower glucose metabolism during sleep. Sleep restriction impairs insulin release, potentially leading to a pre-diabetic state, and during a diet it increases the chance of weight loss from lean body mass rather than fat.

Peter Tripp (1959)

• Tripp experienced 201 hours (8.4 days) of total sleep deprivation, which led to mood instability, confusion, hallucinations, and paranoia, and some psychotic/paranoid features persisted after he resumed normal sleep.

Societal Sleep Deprivation

• The National Sleep Foundation recommends 7-9 hours of sleep for adults, though individual needs vary.

Physiological Effects of Sleep Deprivation

• Hyperactivation of the hypothalamic-pituitary-adrenal (HPA) axis leads to increased stress hormones.
• Oxidative stress and inflammation occur which is decreased ability to detoxify chemical effects of oxygen use and increased markers of inflammation.
• Alterations in cardiovascular function impair blood pressure regulation by increasing heart rate and blood pressure.
• Impaired glucose tolerance and insulin sensitivity increase the risk for diabetes.

Sleep Apnea and Mortality Related to Cardiovascular Diseases

• Sleep apnea severity, decreased sleep time, nocturnal awakenings, and blood oxygen desaturation are associated with an increased risk of cardiovascular problems and mortality.

Two-Process Model of Sleep Regulation

• Homeostatic process (S) accumulates sleep pressure during wakefulness and dissipates it during sleep, is dependent on the length of waking or of previous sleep, with SWS/SWA increasing after sleep deprivation.
• Circadian process (C ) generates circadian rhythms of sleep-wake propensity, is dependent on the internal biological clock, with a strong wake signal in the late afternoon/early evening and a weak wake signal in the early morning.

Interactions Between Processes S and C

• During the daytime, increased sleep pressure is compensated by a strong wake signal from the biological clock.
• In the evening, sleep pressure is high and wake signals decrease, leading to sleepiness.
• At night, sleep pressure decreases and the wake signal remains low, facilitating sleep.
• In the early morning, sleep pressure is low and the wake signal increases, promoting wakefulness.

Phylogeny of Sleep

• Phylogeny is the evolution across species based on heritable traits, this considers changes between groups, not individuals.
• Pre-mammals possibly had a single heterogeneous sleep state, which mixed features of REM in the brainstem and non-REM in the neocortex.
• In early mammals, this state started to split into distinct sleep states: REM and NREM.

Mammals - Sleep Times

• Long sleepers (Mammals): Armadillos (~20 hrs), Opossum (~18hr), Koalas (~15 hrs), Little brown bat (~20hr).
• Short sleepers (mammals): Horses (~2hr of NREM, can sleep standing up).

This covers biological rhythms like circadian and seasonal patterns. It discusses blood pressure peaks, melatonin and cortisol secretion, and the impact on sleep. Early experiments by de Mairon and Kleitman are highlighted.