Topic 1.5 Notes – Sleep

Sleep and the Sleep-Wake Cycle

Consciousness means your awareness of thoughts, feelings, behavior, and what’s happening inside and outside you. That awareness changes. Wakefulness and sleep are two different states of consciousness.

Sleep is:

• biological because body systems regulate it
• active because the brain keeps working in patterned ways
• reversible because you can wake up

Your sleep/wake cycle is a circadian rhythm, a body rhythm that runs about every 24 hours. It helps time:

• sleepiness and alertness
• body temperature
• when you feel ready to sleep or wake

Light is the biggest cue that sets, or entrains, this rhythm. Meals, exercise, and routines also help. Melatonin rises in darkness and helps regulate sleep timing.

A common confusion is sleep pressure versus circadian timing:

• Sleep pressure builds the longer you stay awake.
• Circadian timing creates daily highs and lows in alertness.

So performance can drop because you’ve been awake too long, because your body clock says “sleep now,” or both.

Circadian disruption

• Jet lag happens when you cross time zones and your internal clock no longer matches local time.
• Shift work forces wakefulness during your biological sleep time. Rotating shifts are especially rough because the body keeps having to reset.

Daytime effects often show up on tests as application examples:

• fatigue
• irritability
• slower reaction time
• weaker attention, judgment, learning, and memory

Sleep Stages and Brain-Wave Patterns

Sleep is mainly measured with EEG, which records brain-wave activity. In sleep labs, polysomnography may also include:

• EOG for eye movements
• EMG for muscle tension

Across a typical night, sleep moves through repeating cycles instead of staying in one stage.

Typical sleep cycles across one night

One full sleep cycle lasts about 90 minutes and repeats through the night. Early cycles have more deep NREM Stage 3. Later cycles have more REM.

Wakefulness and NREM stages

• Alert wakefulness shows beta waves
• Relaxed wakefulness shows alpha waves
• Stage 1 NREM shows theta waves
  • light sleep
  • hypnagogic sensations can happen here, like feeling like you’re falling
  • hypnic jerks are sudden body jerks as you drift off
• Stage 2 NREM
  • clearly asleep
  • marked by sleep spindles and K-complexes
• Stage 3 NREM
  • delta waves
  • deep slow-wave sleep
  • hardest to wake from
  • linked to physical restoration
  • somnambulism happens here

EEG patterns in wakefulness and NREM sleep

REM sleep

REM means rapid eye movement sleep. Brain waves look wake-like, but the body is deeply relaxed because of skeletal-muscle atonia. That’s why REM is called paradoxical sleep.

In the night-long sleep graph above, REM periods become longer toward morning.

Key REM facts:

• dreams are most frequent and vivid here
• dreaming can happen outside REM too
• REM periods get longer later in the night
• REM rebound means extra REM after REM deprivation

Dreams and Why Sleep Happens

Dreams are sequences of images, thoughts, emotions, and sensations during sleep.

Theories of dreams

• Activation-synthesis theory says the brain is trying to make sense of internal neural activity, especially during REM.
• Consolidation theory of dreams says dream content reflects memory processing and recent experiences being integrated.

Psychoanalytic dream theory is outside AP exam scope here.

Why sleep occurs

Two main ideas matter, and they work together:

• Memory consolidation strengthens and stabilizes new learning.
• Restoration helps the body and mind recover resources used during the day.

Sleep loss hurts:

• attention
• memory
• decision-making
• reaction time
• mood
• physical performance

People often underestimate how impaired they are when sleep deprived. That gets tested a lot in scenario questions.

Sleep Disorders You Must Know

Treatment and regular sleep schedules can improve daytime functioning and well-being.