Sleep and Biological Clocks

Question 1

What are the biological clocks?

Answer 1

Bio-behavioral rhythms come in multiple time scales

Minutes: respiratory sinus arrhythmia
– changes in heart rate at breathing frequency (e.g., 12 breaths per minute)

Hours: basic-rest activity cycle
– similar to 90 minute sleep cycle

Daily: Circadian rhythm
– e.g., sleep/wake cycle, temperature

Weekly: mood
Monthly: menstrual cycle
Seasonal: mating behaviors, hibernation

Question 2

What are the Circadian Rhythms?

Answer 2

Zeitgeber: stimulus for resetting biological clocks
– light-dark cycle is the zeitgeber for circadian rhythms

Sleep-wake cycles occur without zeitgebers and run in free-running cycles
– humans free-running cycles are around 25 hours
– in animals, free-running cycles are set by internal biological clock in constant environments

Question 3

What are the physiological mechanisms of Circadian Rhythms?

Answer 3

Suprachiasmatic Nucleus (SCN): hypothalamic nucleus above optic chiasm

SCN lesions disrupt rat circadian rhythms, SCN gets retinal input from retinohypothalamic pathway – retinal cells in this pathway are different from rods and cones and sometimes retinally blind people can sync their circadian rhythms by light

Mechanisms of SCN control: via direct neural connections to subparaventricular zone (SPZ; dorsal to SCN)
SCN –> SPZ –> dorsomedial nucleus of the hypothalamus –> sleep-wake controlled brain regions

SCN cells indirectly control circadian rhythms by secreting chemicals carried to other areas of the brain by extracellular fluid

Each SCN cell has its own circadian clock

Protein synthesis feedback loop inhibits clock activity

Question 4

What are seasonal rhythms?

Answer 4

mating, hibernation, and migration: most mammals have a strong link between reproduction and seasonal hormone cycles

SCN lesions stop seasonal mating cycles

Pineal gland secretes melatonin and controls mammalian seasonal rhythms (SCN –> paraventricular nucleus (PVN;hypothalamus) –> spinal cord –> sympathetic NS –> pineal gland

Question 5

How does jet lag and shift work affect circadian rhythms?

Answer 5

Jet lag: Zeitgebers advanced or delayed
shift work: Zeitgebers don’t change
– light exposure at appropriate times can help adjust to jet lag and shift work

melatonin affects SCN sensitivity to zeitgebers and alters circadian rhythms
– pre-bedtime melatonin can ease jet lag and shift changes

Question 6

What is sleep?

Answer 6

Sleep stages
Stage 1: transition from wakefulness to sleep
– theta waves (3.5-7.5 Hz)
– similar to alpha waves, but slower and eyes open and close, roll up and down

Stage 2: theta with spindles (short bursts of fast activity) and K-complexes (sharp up-downward deflections)
– spindles: memory consolidation
– K-complexes: precursor of delta waves and only in stage 2

Stage 3: delta waves (1-3 Hz)
– slow-wave sleep (SWS): deepest sleep

REM sleep: 90min after sleep onset, last 20-30 mins in 90 min cycles
– beta with Theta waves cause increased cerebral blood flow (cortical activity)
– rapid eye movement, loss of muscle tone, increased respiration, blood pressure, and heart rate, vivid dreams, sexual arousal