Sleep is vital for both physical and mental health. It allows the body to rest and restore itself. Sleep occurs in cycles that repeat throughout the night. The first half of sleep is made up of lighter stages of non-rapid eye movement (NREM) sleep. During this time, the body begins to relax and brain wave activity starts to slow down. However, people can still be easily awakened. As sleep progresses, brain wave activity continues to slow and the body becomes immobile. Let’s take a look at what happens during the first half of a typical night’s sleep.
The Hypnagogic State
When first falling asleep, people enter the hypnagogic state. This transitional period between wakefulness and sleep lasts just a few minutes. During this time, people may experience hallucinations and dream-like images. Muscle contractions may also occur. The hypnagogic state marks the beginning of stage 1 sleep.
Characteristics of the Hypnagogic State
- Lasts less than 15 minutes
- Hallucinations and dream-like images
- Muscle contractions or twitches
The hypnagogic state represents the transition between wakefulness and light sleep. Dreams and visions can occur as the brain begins to relax. This state only lasts for a few minutes before stage 1 sleep begins.
Stage 1 Sleep
Stage 1 is the lightest stage of NREM sleep. It lasts for 5-10 minutes and accounts for 2-5% of total sleep time. During this stage, the muscles relax, heart rate slows, and brain wave activity begins to change.
Characteristics of Stage 1 Sleep
- Lasts 5-10 minutes
- Beginning of light sleep
- Relaxed muscles
- Slower heart rate
- Brain waves start to slow from alpha to theta waves
People are still somewhat aware of their surroundings in stage 1 sleep. Sudden noises or movements can wake them up. As stage 1 progresses, the brain prepares to enter deeper sleep.
Stage 2 Sleep
Stage 2 is considered the first stage of true light sleep. It lasts for 10-25 minutes and represents about 45-55% of total sleep. Brain wave activity continues to slow, and eye movements stop. Body temperature starts dropping and heart rate slows further.
Characteristics of Stage 2 Sleep
- Lasts 10-25 minutes
- Increased wave amplitude and slower frequency compared to stage 1
- No eye movements
- Decreased body temperature
- Slower heart rate
During stage 2, the body prepares for deep, restorative sleep. Blood flow is directed away from the muscles and towards the body’s tissues to help with repair. The immune system also becomes more active.
Stages 3 and 4: Slow Wave Sleep
The next stages of NREM sleep are known as slow wave sleep because brain waves become even slower.
Stage 3 Sleep
Stage 3 used to be considered the beginning of deep sleep. It lasts for 20-40 minutes. Large, slow brain waves called delta waves begin to emerge. Muscles are relaxed and breathing slows. Blood pressure drops and heart rate reaches its lowest point.
Characteristics of Stage 3 Sleep
- 20-40 minutes in length
- Beginnings of slow wave sleep
- Relaxed muscles and slowed breathing
- Delta brain waves intermixed with smaller, faster waves
- Difficult to wake the person
Stage 3 brings the brain and body closer to a state of deep, restorative sleep. The initial delta brain waves help transition the brain into the deepest stage of sleep.
Stage 4 Sleep
Stage 4 used to represent deep sleep. It lasts for 30 minutes or longer during the first sleep cycles. Delta brain waves dominate this stage. Breathing is very slow and steady. Blood pressure and heart rate reach their lowest levels. Muscles are fully relaxed and it becomes very difficult to wake someone.
Characteristics of Stage 4 Sleep
- 30+ minutes in length
- Very deep sleep
- Brain dominated by delta waves
- heartbeat and respiration slow dramatically
- Muscles completely relaxed
- Very difficult to wake the person
Stages 3 and 4 allow the body to completely relax and recharge. Hormones are released to facilitate growth and repair. The deepest sleep happens during these stages.
Updated Sleep Stage Classifications
In 2007, the American Academy of Sleep Medicine updated its definitions of the sleep stages. Stages 3 and 4 were combined into a single stage of slow wave sleep. The current classifications are:
- Stage 1 – Light sleep
- Stage 2 – True light sleep
- Stage 3 – Slow wave sleep
- REM sleep
While stages 1-4 are still sometimes referred to, the updated system uses the three stages of NREM sleep. Slow wave sleep encompasses the deepest, most restorative delta wave sleep.
Time Spent in Each Stage
During a typical night’s sleep, the time spent in each sleep stage is:
| Sleep Stage | Time Spent |
|---|---|
| Stage 1 | 5-10 minutes |
| Stage 2 | 45-55% of total sleep time |
| Slow Wave Sleep | 15-25% of total sleep time |
| REM Sleep | 20-25% of total sleep time |
The amount of time spent in each stage can vary significantly between individuals. With age, the time spent in deep, slow wave sleep decreases.
Sleep Cycles
Sleep occurs in 90-120 minute cycles throughout the night. Each cycle progresses through the stages of sleep ending in REM sleep. The cycles repeat 3-5 times per night.
Composition of Sleep Cycles
| Stage | Percentage of Sleep Cycle |
|---|---|
| NREM Stages 1-2 | 5-15% |
| Slow Wave Sleep | 70-80% |
| REM Sleep | 20-25% |
Slow wave sleep dominates the first third of the night. REM sleep occurs more frequently in the second half of the night. The percentage of each stage changes across the sleep cycles.
Changes Across Cycles
As sleep progresses through the night, here are the key changes that occur:
- Stage 1 shortens with each cycle
- Stage 2 stays relatively constant
- Slow wave sleep decreases with each cycle
- REM periods lengthen with each cycle
The deep sleep that occurs in slow wave sleep is greatest during the first two cycles. REM sleep increases towards morning to facilitate dreaming.
Summary of the First Half of Sleep
During the first half of a night’s sleep, a person transitions from light sleep to deep, slow wave sleep. Key characteristics include:
- Fall asleep by entering the hypnagogic state followed by stage 1 sleep
- Reach stage 2 light sleep after about 10-15 minutes
- Progress through stages 3 and 4 slow wave sleep
- Each cycle lasts about 90-120 minutes
- Slow wave sleep dominates the first 3-4 cycles
Getting adequate slow wave sleep is important for body restoration. As sleep continues through the night, REM sleep becomes more prominent. But the first half of sleep largely focuses on descending into deep, restorative stages. Understanding the stages of sleep can help ensure you get truly restful, high-quality sleep each night.
Conclusion
The first half of sleep is marked by lighter stages at the beginning followed by deep, slow wave sleep. Falling asleep takes people through the hypnagogic transitional state leading into stage 1. Stage 2 establishes true light sleep. Stages 3 and 4 represent very deep sleep characterized by delta brain waves. Each cycle lasts about 90-120 minutes and repeats 3-5 times through the night. Slow wave sleep dominates the first half of sleep to allow physical and mental restoration. Getting enough deep sleep is vital for health, immunity, brain function, and wellbeing. Understanding your personal sleep cycles can help improve your sleep quality.