What is consciousness?
Consciousness is being aware of yourself and the world around you: your thoughts, feelings and senses. It is not all-or-nothing. You can be fully alert, day-dreaming, drowsy, asleep, or in an altered state (for example under hypnosis or a drug).
Attention works like a torch beam. You notice only a small part of what is around you. Much of the brain's work happens outside awareness, like keeping balance or understanding a familiar word. Psychologists call this dual processing: a fast automatic track and a slower conscious track.
Circadian rhythm: your body clock
A circadian rhythm is a body cycle of about 24 hours. Body temperature, alertness and many hormones rise and fall on this clock.
Light hits the eyes and signals a small cluster of cells in the hypothalamus, the suprachiasmatic nucleus (SCN). In darkness, the SCN tells the pineal gland to release melatonin, which makes us sleepy. Bright screens late at night can delay melatonin and push sleep later.
Teenagers' clocks naturally shift later, so they feel sleepy later at night. Jet lag and night shift work happen when the clock and the real time do not match.
Stages of sleep and the 90-minute cycle
An EEG (electroencephalogram) records brain waves. Awake and alert: fast, small beta waves. Relaxed with eyes closed: alpha waves.
- N1: light sleep, a few minutes. Theta waves. You may feel like you are falling or see dream-like images (hypnagogic sensations).
- N2: true sleep. Bursts of fast waves called sleep spindles and large K-complexes. We spend the most time here.
- N3: deep, slow-wave sleep with big delta waves. Hard to wake. Growth hormone is released; the body repairs itself.
- REM (rapid eye movement): the brain is very active, the eyes dart, most vivid dreams happen, but the big muscles are relaxed, almost paralysed. It is called paradoxical sleep.
One cycle (N1 → N2 → N3 → N2 → REM) takes about 90 minutes. We have 4–5 cycles a night. Early cycles have more N3; later cycles have longer REM periods.
Why we sleep and why we dream
Sleep helps us: restore the body and clear waste from the brain, consolidate memories (fix the day's learning), support growth, and save energy. Many teenagers need 8–10 hours.
Sleep deprivation harms attention, mood, memory, immune health and reaction time; drowsy driving causes many accidents. After losing REM sleep, people show REM rebound: extra REM the next night.
Why do we dream? Ideas include:
- Activation-synthesis: the brainstem fires random signals in REM and the cortex weaves them into a story.
- Consolidation / information-processing: dreams help sort and store the day's memories.
- Wish-fulfilment (Freud): dreams show hidden wishes. This is hard to test and has little scientific support.
Sleep disorders
- Insomnia: regular trouble falling or staying asleep.
- Narcolepsy: sudden attacks of sleep during the day, sometimes going straight into REM.
- Sleep apnoea: breathing stops again and again during sleep; often loud snoring and daytime tiredness.
- Sleepwalking (somnambulism) and night terrors: happen in deep N3, mostly in children; the person usually remembers nothing.
- REM sleep behaviour disorder: the muscles are not relaxed in REM, so the person acts out dreams.
Good habits (sleep hygiene): a fixed bedtime, morning daylight, no screens or caffeine late, a dark and cool room.
Key formulas and definitions
- Circadian rhythm ≈ 24 hours; light → SCN → melatonin at night
- EEG: beta (alert) → alpha (relaxed) → theta (N1) → spindles (N2) → delta (N3)
- One sleep cycle ≈ 90 minutes: N1 → N2 → N3 → N2 → REM
- Cycles per night ≈ hours slept × 60 ÷ 90
- REM = active brain + relaxed muscles + vivid dreams (paradoxical sleep)
Worked examples
1. Riya sleeps from 11 pm to 6:30 am. About how many full sleep cycles does she get?
7.5 hours = 450 minutes. 450 ÷ 90 = 5 cycles.
2. A sleep lab shows large, slow delta waves and the person is very hard to wake. Which stage is this?
N3, deep slow-wave sleep.
3. A student who missed REM sleep for two nights sleeps normally on the third night. What change is expected?
REM rebound: more time than usual in REM sleep that night.
4. A traveller flies from Delhi to London (4.5 hours behind) and wakes at 3 am local time, wide awake. Explain.
Her circadian rhythm is still on Delhi time, where it is 7:30 am. Her body clock has not yet reset to the new light-dark cycle: jet lag.
Common mistakes
- Thinking the brain switches off in sleep. In REM it is nearly as active as when awake.
- Saying dreams happen only in REM. They happen most vividly in REM but can occur in NREM too.
- Mixing up the order of EEG waves. Faster and smaller when alert (beta), slower and bigger in deep sleep (delta).
- Believing sleepwalking happens in REM. It happens in deep N3, when the muscles can still move.