📖 Reading 2.1: Circadian Rhythm and Sleep Pressure
📖 Reading 2.1: Circadian Rhythm and Sleep Pressure
Introduction
Sleep does not occur through a single switch.
It develops through the interaction of biological timing, accumulated sleep need, brain activity, light exposure, behavior, physical health, thoughts, emotions, relationships, and environment.
One of the most useful scientific frameworks for understanding this process is the two-process model of sleep regulation.
The model describes two major forces:
Process S: homeostatic sleep pressure
Process C: circadian timing
These two processes help explain why a person can be exhausted yet unable to sleep, why late naps may affect bedtime, why jet lag feels disorienting, and why a regular waking rhythm can be helpful.
The model is not a diagnosis or complete theory of every sleep difficulty. It is a map of part of God-created human physiology.
Process S: The Pressure for Sleep
The body has a biological need for sleep that generally increases during wakefulness.
Sleep researchers call this the homeostatic sleep process, or Process S.
Homeostasis refers to the body’s effort to maintain internal stability. In sleep regulation, Process S reflects the growing need for recovery after wakefulness.
As the waking period continues, sleep pressure usually increases. During sleep, the pressure gradually decreases.
The two-process model has remained a foundational framework in sleep science for more than four decades and continues to help researchers understand sleep timing and intensity. (PubMed)
Adenosine and Sleep Pressure
Adenosine is one chemical associated with the accumulation of sleep pressure.
While a person remains awake, adenosine levels increase in parts of the brain. During sleep, this pressure is reduced. Caffeine can temporarily interfere with the signaling of sleep pressure by blocking adenosine receptors. (NHLBI, NIH)
This does not mean that adenosine alone explains sleep. Human sleep regulation involves many systems.
Still, it helps explain why caffeine may make a person feel more alert without eliminating the underlying need for sleep.
Caffeine does not create sleep.
It may temporarily mask part of the experience of sleepiness.
What Can Reduce Sleep Pressure?
Sleep itself reduces sleep pressure.
Naps can also reduce some accumulated pressure. The effect depends on factors such as:
Length
Timing
Previous sleep
Age
Individual sensitivity
Health
Circadian timing
A brief nap may help some people function safely and effectively.
A long or late nap may make it more difficult for another person to feel sleepy at the intended bedtime.
The proper question is not:
Are naps always good or always bad?
The better question is:
How does this particular nap affect this person’s nighttime sleep and daytime functioning?
People with shift work, illness, pregnancy, caregiving demands, or significant sleep loss may need different strategies from someone with a stable daytime schedule.
Individualized medical concerns should be discussed with qualified professionals.
Process C: The Circadian Timing System
The second major process is circadian timing, or Process C.
Circadian rhythms are repeating biological patterns that occur across approximately twenty-four hours.
The human circadian system influences:
Alertness
Sleepiness
Body temperature
Hormone release
Digestion
Metabolism
Immune function
Many other bodily activities
A central circadian clock in the brain helps coordinate timing across the body. Light and darkness are especially important environmental signals, though activity, meals, schedules, and social patterns can also contribute. (NHLBI, NIH)
The circadian system does not merely make a person increasingly tired as the day continues.
It also provides changing signals for alertness and sleep across the day and night.
This helps explain why a person may struggle to sleep several hours before the usual bedtime despite being tired.
The circadian system may still be promoting wakefulness.
The Two Processes Work Together
Sleep becomes more likely when:
Homeostatic sleep pressure is sufficiently high
Circadian timing is supportive of sleep
The environment and organic human are able to settle
Wakefulness becomes more likely when:
Sleep pressure has been reduced
Circadian timing supports alertness
Environmental or internal signals promote vigilance
This interaction helps explain several familiar experiences.
The Late Nap
A late nap may reduce sleep pressure shortly before the intended bedtime.
The person may then lie down at the usual hour but feel less sleepy.
The Second Wind
Sleep pressure may be high, but the circadian system may still provide an alerting signal. A person may feel temporarily more energetic in the evening.
Jet Lag
The local clock changes quickly, but the circadian system does not immediately follow. The body may promote sleep or alertness at times that no longer fit the new environment.
Shift Work
A night worker may need to remain alert when the circadian system is supporting sleep and attempt sleep during daylight when the system is supporting wakefulness.
Sleeping Earlier Than Usual
A person may go to bed much earlier after one difficult night. Sleep pressure may be elevated, but circadian timing may not yet support sustained sleep.
The person may fall asleep briefly, awaken, and then fear that something is wrong.
These experiences do not mean the person is defective.
They reveal that sleep has timing.
God Created Humans Within Time
Genesis describes creation through repeated rhythms:
“There was evening, and there was morning.”
The sun, moon, and lights in the heavens mark days and seasons.
Psalm 104:19 says:
“He appointed the moon for seasons.
The sun knows when to set.”
The Bible does not describe circadian biology in scientific terms. Scripture gives the theological meaning of a world ordered by God.
Sleep science describes some of the bodily processes within that ordered creation.
The Christian does not need to choose between:
God created rhythms.
and
Human beings have circadian timing systems.
The scientific description concerns how part of creation functions.
The theological confession concerns who created, sustains, and gives meaning to it.
Light as a Timing Signal
Light entering the eyes communicates with the central circadian timing system.
Morning and daytime light can help reinforce a daytime signal. Bright light during the late evening or night may delay sleep timing in some people.
The influence depends on:
Timing
Intensity
Duration
Individual sensitivity
Previous light exposure
Age
Eye health
Work schedule
The principle should not be oversimplified into:
All light is bad at night.
People need light for safety, caregiving, employment, navigation, and ordinary life.
The practical goal is to use light thoughtfully.
Morning daylight is often a freely available timing signal. People who cannot safely go outside may sit near a bright window where possible.
Night workers and people with circadian rhythm disorders may require specialized guidance. They should not assume that ordinary advice for daytime workers automatically fits their schedule.
Chronotype: Earlier and Later Timing
Not every person’s internal timing is identical.
Some people naturally tend to become sleepy earlier and wake earlier.
Others tend to become sleepy later and wake later.
These tendencies are commonly described as chronotype.
Chronotype is shaped by biological and environmental influences. Age also matters. Teenagers and younger adults commonly shift toward later timing, while many older adults shift toward earlier timing. (NHLBI, NIH)
Chronotype should not become a moral label.
An earlier chronotype is not more disciplined or spiritual.
A later chronotype is not automatically lazy.
At the same time, chronotype does not eliminate responsibility. A person still needs to make wise decisions about work, school, caregiving, worship, light exposure, and sleep opportunity.
The challenge often arises when internal timing and social obligations conflict.
A later-timing person may need to wake early for work.
An earlier-timing person may need to remain awake for evening responsibilities.
This conflict is sometimes called circadian misalignment or social jet lag.
Sleep Stages and Cycles
After sleep begins, the brain and body move through recurring stages.
Sleep is broadly divided into:
Non-rapid eye movement sleep
Rapid eye movement sleep
Non-rapid eye movement sleep includes three stages.
Stage N1
N1 is the transition from wakefulness into sleep.
A person may drift in and out and may not always realize sleep has begun.
Stage N2
N2 is established sleep.
Brain activity changes, body temperature decreases, and awareness of the environment becomes reduced.
Adults commonly spend a large portion of the night in N2 sleep.
Stage N3
N3 is often called deep sleep or slow-wave sleep.
More deep sleep generally occurs earlier in the night.
REM Sleep
REM means rapid eye movement.
Brain activity becomes more similar in some ways to waking activity. Vivid dreaming frequently occurs during REM sleep, though dreams can occur during other stages.
The muscles normally become less active during REM, helping prevent most dream enactment.
A typical night includes recurring cycles of non-REM and REM sleep. The cycles commonly repeat about every eighty to one hundred minutes, with approximately four to six cycles in many nights. Brief awakenings may occur between cycles. Deep sleep is usually more prominent earlier, while REM sleep becomes more prominent later. (NHLBI, NIH)
These are general patterns, not a nightly performance target.
Brief Awakenings Are Not Always Abnormal
People sometimes believe that healthy sleep means entering unconsciousness and remaining completely unaware until morning.
Human sleep is usually more dynamic.
A person may briefly awaken between cycles, adjust position, hear a sound, or become aware of the room.
The awakening may be so brief that it is not remembered.
The problem often begins when the awakening is interpreted as danger:
I should not be awake.
Something is wrong.
I must get back to sleep immediately.
The body may become more alert because the person is now monitoring, calculating, and worrying.
Understanding sleep cycles can help a participant respond:
Awakening is not automatically an emergency.
I can remain calm and allow the next period of sleep to come.
Persistent or prolonged awakenings may still deserve attention, especially when accompanied by daytime impairment, pain, breathing problems, nightmares, or other concerning symptoms.
Why the First Part and Last Part of Night Differ
Deep sleep is generally more concentrated earlier in the sleep period.
REM sleep generally becomes more prominent later.
This helps explain why cutting sleep short at either end may affect the balance of sleep stages.
However, participants should not try to manage sleep stages directly.
You cannot consciously choose:
I will enter N3 now.
Consumer trackers may estimate sleep stages, but they are not equivalent to a professionally conducted sleep study. The American Academy of Sleep Medicine has warned that consumer sleep technologies should not be used independently to diagnose or treat sleep disorders. (AASM)
Trackers may offer broad observations, but their stage estimates can differ meaningfully from polysomnography, the clinical reference standard. (PubMed)
The goal is not to chase a perfect stage chart.
The goal is to provide a reasonable sleep opportunity and address concerns wisely.
Circadian Misalignment Is Not a Moral Failure
Some participants live with schedules that conflict with their biology.
Examples include:
Night-shift workers
Rotating-shift workers
Parents of infants
Caregivers
Military personnel
Emergency workers
Transportation workers
Agricultural workers
Students
People crossing time zones
People living in regions with extreme seasonal daylight
People whose employment hours change unpredictably
The course should not shame these participants with advice designed only for people who control every hour.
A stable wake time and regular schedule may be helpful, but not every person has equal power to establish them.
Christian compassion asks:
What rhythm is realistically available within this person’s circumstances?
Practical Two-Process Questions
When sleep timing is difficult, ask:
Sleep Pressure
How long have I been awake?
Did I nap?
Was the nap long or late?
Have I been unusually inactive?
Have I used caffeine?
Am I recovering from previous sleep loss?
Circadian Timing
When do I normally become sleepy?
When did I receive daylight?
Have I been exposed to bright light late at night?
Has my schedule shifted?
Am I traveling or working at night?
Am I trying to sleep much earlier than usual?
Other Organic-Human Factors
Am I in pain?
Am I worried or angry?
Do I feel safe?
Is my breathing disrupted?
Is the environment too hot, cold, bright, or noisy?
Am I experiencing hormonal changes?
Is a medication or substance involved?
Do I need qualified evaluation?
The two-process model helps organize questions. It does not answer all of them.
No-Cost Practice: Notice Your Sleepiness Window
For one week, notice:
When you first feel a meaningful increase in sleepiness
Whether you continue past that period
Whether a second wind appears
When you naturally feel most alert
When daylight enters your day
Whether naps influence nighttime sleep
Do not force an immediate change.
First learn the pattern.
Then choose one realistic adjustment.
Global Adaptation
People living near the equator, far from the equator, in urban environments, or in rural settings may experience very different light conditions.
People may also have cultural patterns involving:
Midday rest
Late evening meals
Multigenerational households
Communal sleeping
Early agricultural work
Evening worship
Nighttime employment
Seasonal changes in work
The course should honor these realities.
The goal is not to impose one culturally narrow clock schedule.
The goal is to understand the interaction between internal timing, environmental signals, responsibilities, and sleep opportunity.
What the Evidence Does and Does Not Show
The two-process model is a powerful conceptual framework supported by decades of sleep research. It explains the interaction between homeostatic sleep pressure and circadian timing. (PubMed)
The evidence supports that light, darkness, waking duration, sleep, caffeine, scheduling, age, and individual differences influence sleep timing. (NHLBI, NIH)
The evidence does not show that:
Two processes explain every sleep disorder
Everyone has the same ideal bedtime
Later chronotypes are morally undisciplined
Earlier chronotypes are spiritually mature
Every awakening is abnormal
Sleep stages should be controlled consciously
Consumer trackers can independently diagnose disorders
One late nap always ruins sleep
Perfect regularity is possible for every person
Understanding the model guarantees better sleep
Research provides a map.
It does not give complete control.
Ministry Role and Safety
A minister, chaplain, or Christian life coach may help a participant:
Understand basic sleep timing
Notice the difference between tiredness and sleep readiness
Reflect on schedule and light patterns
Reduce shame connected with chronotype
Develop one realistic rhythm
Prepare questions for a healthcare professional
A ministry leader should not:
Diagnose a circadian rhythm disorder
Prescribe clinical light therapy
Recommend medication or melatonin dosages
Interpret tracker-generated sleep stages as a diagnosis
Tell a shift worker to change employment without understanding the situation
Require a schedule that creates caregiving or occupational danger
A person who cannot remain awake safely, becomes sleepy while driving, experiences severe impairment, or has persistent sleep-timing problems should seek qualified help.
Reflection Questions
How would you explain the difference between sleep pressure and circadian timing?
When does your sleepiness usually increase?
Have you noticed an evening second wind?
How do naps affect your nighttime sleep?
Does your natural timing tend to be earlier, later, or somewhere between?
Where does your schedule conflict with your organic body?
Do you interpret brief awakenings as ordinary, frustrating, or dangerous?
Has a sleep tracker helped you notice a pattern, or has it increased anxiety?
What is one no-cost way to support your circadian rhythm?
Which concern may require professional rather than ministry support?
Closing Prayer
Creator God, thank you for designing my body with rhythms of waking and sleeping.
Help me understand my organic body without turning knowledge into control or measurement into condemnation.
Give me wisdom to notice sleep pressure, light, timing, activity, and the realities of my daily life.
Help me honor the responsibilities I carry while also receiving my creaturely limits.
Protect those who work at night, care for others, travel, or live with schedules they cannot easily change.
Holy Spirit, guide me toward one faithful practice and toward qualified help when needed.
In Jesus’ name, amen.
Academic and Ministry References
Borbély, A. A. (2022). The two-process model of sleep regulation: Beginnings and outlook. Journal of Sleep Research, 31(4), e13598. (PubMed)
Borbély, A. A., Daan, S., Wirz-Justice, A., and Deboer, T. (2016). The two-process model of sleep regulation: A reappraisal. Journal of Sleep Research, 25(2), 131–143. (PubMed)
Khosla, S., Deak, M. C., Gault, D., et al. (2018). Consumer sleep technology: An American Academy of Sleep Medicine position statement. Journal of Clinical Sleep Medicine, 14(5), 877–880. (AASM)
National Heart, Lung, and Blood Institute. (2022). How sleep works: Sleep phases and stages. (NHLBI, NIH)
National Heart, Lung, and Blood Institute. (2022). How sleep works: Your sleep/wake cycle. (NHLBI, NIH)
Schyvens, A. M., Van Oost, N. C., Aerts, J. M., et al. (2024). Accuracy of Fitbit Charge 4, Garmin Vivosmart 4, and WHOOP versus polysomnography: Systematic review. Journal of Medical Internet Research, 26, e52192. (PubMed)
Scripture References Used
Genesis 1:1–19
Psalm 19:1–6
Psalm 74:16
Psalm 90:12
Psalm 104:19–23
Psalm 121:1–8
Ecclesiastes 3:1–8