The science of sleep: how sleep affects your brain and body

Sleep plays a critical role in physical health, mental wellbeing and everyday functioning. Yet many Australians report not getting enough sleep or waking feeling unrefreshed, even after a full night's rest.¹ Poor sleep can affect mood, concentration, memory and overall quality of life. Understanding what happens when you sleep – including the sleep-wake cycle, REM and deep sleep, and the effects of sleep debt – can help you make the right changes to support better sleep and overall wellbeing.

What happens when you sleep?

During sleep, the brain processes information from the day, stores memories, regulates emotions and supports learning. The body also repairs tissues, regulates hormones and supports immune system function.^1,2 

Sleep is an active biological process that allows the brain and body to recover, regulate essential functions and prepare for the next day. While it may feel like the body is "switching off", important restorative processes continue throughout the night.³ 

Sleep is essential for maintaining physical health, cognitive performance and emotional wellbeing.^1,2 

What is the sleep-wake cycle?

The sleep-wake cycle is the body's internal 24-hour clock that helps regulate sleep patterns, alertness and other biological processes throughout the day.⁴

Also known as the circadian rhythm, the sleep-wake cycle responds to environmental cues such as light and darkness. Exposure to natural light helps signal wakefulness, while darkness encourages the release of melatonin, a hormone that helps prepare the body for sleep.⁴

The timing of your sleep is influenced by both your body's need for sleep and your internal body clock. Disruptions to the sleep-wake cycle — such as shift work, jet lag or irregular sleep schedules — can affect sleep quality and overall health.⁴

What are the stages of sleep?

Sleep occurs in a series of stages that repeat throughout the night in cycles of approximately 90 minutes.⁵ Each stage plays a different role in supporting physical recovery, brain function and overall wellbeing.

The stages of sleep include:⁶

• Stage 1 (N1): Light sleep – the transition between wakefulness and sleep.

• Stage 2 (N2): Light sleep – a deeper stage of sleep where body temperature drops, muscles relax, and heart rate and breathing slow.

• Stage 3 (N3): Deep sleep – important for physical recovery, growth, immune function and restorative sleep.

• Stage 4 (N4): REM sleep, associated with dreaming, memory, learning and creativity.

The body cycles through these stages multiple times throughout the night, with each stage contributing to restorative sleep.

How much sleep do adults need?

Most adults need between 7 and 9 hours of sleep per night to support physical health, cognitive performance and wellbeing.^2,7

Sleep needs can vary between individuals, with some people naturally requiring slightly more or less sleep than others. However, regularly getting less sleep than your body needs may affect mood, concentration, reaction time and overall health.^2,3

Getting enough sleep is just as important as sleep quality. Consistently achieving the amount of sleep your body needs can help support daytime functioning and recovery.^1,2

What is REM sleep?

REM (rapid eye movement) sleep is a stage of sleep most closely associated with dreaming.⁵

During REM sleep, people experience vivid, structured dreams while many of the body's muscles relax completely (Source: E). REM sleep occurs in short episodes throughout the night, with longer dream periods typically occurring in the morning hours.⁵

REM sleep is also thought to play an important role in memory consolidation, emotional processing and learning, making it an essential component of restorative sleep.⁵

Most dreaming occurs during REM sleep, although dreams can occur during other stages of sleep as well.⁵ 

What is deep sleep?

Deep sleep is the stage of sleep most closely associated with physical recovery and restoration.⁶

Deep sleep, also known as slow-wave sleep, occurs predominantly during the first part of the night. It is considered the deepest stage of sleep and is more difficult to wake from than lighter stages of sleep.^3,6

Research suggests deep sleep plays an important role in helping the body recover and restore itself.⁶

What happens when you don't get enough sleep?

Not getting enough sleep can affect physical health, mental wellbeing and day-to-day functioning.¹

Short-term effects may include: impaired concentration and memory, increased irritability and mood changes, reduced reaction time, and difficulty making decisions. Ongoing sleep deprivation has been linked to an increased risk of several chronic health conditions.¹

Sleep plays an important role in many processes throughout the body. Research has linked ongoing sleep problems with an increased risk of several chronic health conditions, while insufficient sleep may also affect mood, concentration and overall wellbeing.¹

Because sleep affects many different systems throughout the body, consistently getting enough quality sleep is important for both short-term and long-term health.

What is sleep debt?

Sleep debt refers to the deficit between the amount of sleep your body needs and the amount of sleep you actually get.⁸

For example, if your body needs eight hours of sleep each night but you only get six hours, you accumulate two hours of sleep debt. Over time, this sleep debt can affect alertness, mood, cognitive performance, and physical health.^1,8 

Many people attempt to catch up on sleep by sleeping longer on weekends. While extra sleep may help reduce some effects of short-term sleep loss, it may not fully reverse the impact of ongoing sleep deprivation.⁸

Maintaining a consistent sleep schedule is generally more beneficial than regularly relying on catch-up sleep.^6,8

What causes poor sleep quality?

Poor sleep quality can be influenced by a range of lifestyle, behavioural and environmental factors. Irregular sleep schedules, caffeine later in the day, alcohol, screen use before bed, stimulating evening activities and an uncomfortable sleep environment can all make it harder to fall asleep or stay asleep.⁷

Stress and racing thoughts may also affect sleep for some people. Developing a regular sleep routine, reducing distractions before bed and creating a comfortable sleep environment may help improve sleep quality over time.⁷

What is sleep hygiene and how can it improve sleep quality?

Sleep hygiene refers to the habits, behaviours and environmental factors that support healthy sleep.⁷ Good sleep hygiene can help improve sleep quality, make it easier to fall asleep and support overall health and wellbeing.

While occasional sleep difficulties are common, consistently practising healthy sleep habits may help improve sleep over time.

Evidence-based sleep hygiene recommendations include:⁷

• Maintaining a regular sleep schedule
• Limiting caffeine later in the day
• Reducing screen exposure before bed
• Creating a comfortable sleep environment
• Getting regular exposure to natural daylight
• Engaging in regular physical activity 

Small changes to your daily routine can help improve sleep quality over time.

When should you speak to a healthcare professional in Australia about sleep concerns?

If you're struggling with sleep and finding it’s affecting your daily life, it may be worth speaking with an AHPRA-registered healthcare professional if you:

• Find it difficult to fall asleep on a regular basis
• Frequently wake during the night
• Experience ongoing daytime fatigue
• Snore or have breathing difficulties during sleep
• Your ability to work, study or carry out daily activities is affected

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Whether you're struggling to fall asleep, waking during the night or feeling tired throughout the day, speaking with an AHPRA-registered clinician may help you explore alternative healthcare options that are appropriate for your circumstances.

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Disclaimer

Alternaleaf provides alternative healthcare services only. Eligibility for treatment is determined by a registered medical practitioner during a clinical assessment. ‍

Frequently asked questions about sleep

How many hours of sleep do adults need each night?

Most adults need between 7 and 9 hours of sleep per night to support physical health, cognitive performance and overall wellbeing. Sleep needs can vary between individuals, but regularly getting less sleep than your body requires may affect mood, concentration, reaction time and long-term health.^1-3,7

What is the difference between REM sleep and deep sleep?

‍REM sleep and deep sleep serve distinct roles within the sleep cycle. Deep sleep is most closely associated with physical recovery, tissue repair and immune support, while REM sleep supports memory consolidation, emotional processing and learning. Both stages are essential, and disruptions to either can affect overall sleep quality.^3,5,6

What causes poor sleep quality?

‍Poor sleep quality can result from a range of lifestyle, behavioural and environmental factors. Common contributors include irregular sleep schedules, caffeine consumed late in the day, screen use before bed, and stress. Developing a consistent pre-sleep routine and reducing stimulating activity in the evening may help improve sleep quality over time.⁷ 

What is sleep debt, and can you recover from it?

‍Sleep debt is the cumulative gap between the amount of sleep your body needs and the amount it actually gets. Sleeping longer on weekends may relieve some short-term fatigue, but research suggests it does not fully reverse the effects of ongoing sleep loss. Maintaining a consistent sleep schedule is generally more effective than relying on catch-up sleep.^1,6,8

When should I speak to a healthcare professional about sleep concerns?

‍Consider speaking with an AHPRA-registered healthcare professional if sleep difficulties are regularly affecting your mood, concentration or ability to carry out daily activities. This includes frequently struggling to fall asleep, waking during the night without returning to sleep, or experiencing ongoing daytime fatigue that does not improve with better sleep habits.