Introduction
Night-time investigations are often treated as if darkness itself makes people more perceptive. Human biology suggests a more careful interpretation. The circadian system coordinates daily rhythms in sleep propensity, body temperature, hormone secretion and alertness. These rhythms continue even when a person tries to remain awake, and they are influenced by light exposure, sleep timing, work schedules and individual differences.
Understanding chronobiology helps investigators distinguish environmental events from predictable changes in attention and performance. It also supports safer scheduling.
The biological clock
The central circadian pacemaker is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. It receives timing information from specialised retinal cells that respond to light. The SCN coordinates rhythms across the body, including signals that influence melatonin secretion and sleep-wake timing.
Melatonin is commonly described as a “darkness signal.” Its timing is influenced by the light-dark cycle. It is not a simple sleep switch, and its presence does not mean a person is unconscious or unable to perceive events.
Circadian phase differs among individuals. Chronotype, age, habitual schedule and recent light exposure all influence when a person naturally feels alert or sleepy.
Alertness across the night
Alertness is shaped by at least two interacting processes:
- Circadian timing: the internal clock promotes wakefulness or sleep at different times of day.
- Homeostatic sleep pressure: the longer a person remains awake, the greater the drive for sleep.
These processes are related but distinct. A person may feel temporarily more alert because of stimulation or activity while still experiencing impaired attention and reaction time. The biological night often includes a period of reduced alertness, but exact timing varies.
This article focuses on circadian timing rather than sleep deprivation itself. Even a well-rested investigator can experience a circadian low if the session occurs at an unfavourable biological time.
Light, darkness and perception
Light affects circadian timing, especially when exposure occurs at biologically sensitive times. Bright light at night can shift the clock and suppress melatonin, depending on intensity, spectrum, duration and timing. Darkness may reduce visual information, making ambiguous shapes and movements harder to interpret.
The important distinction is that darkness changes the available sensory evidence; it does not establish that unusual events become more frequent. Low illumination can increase uncertainty and reliance on expectation.
Designing a responsible night session
A field schedule should record:
- Local clock time and time zone
- Participant's usual sleep schedule, if voluntarily provided
- Recent shift work or travel across time zones
- Lighting conditions and duration of exposure
- Breaks, food and hydration
- Session duration and any signs of reduced alertness
Avoid making medical claims about participants. The aim is to understand schedule-related variables, not to diagnose circadian disorders.
AERM field protocol
- Plan sessions with adequate rest and avoid unnecessarily long overnight work.
- Record start and finish times and the lighting environment.
- Use neutral witness interviews before discussing other people's interpretations.
- If an event is reported, note the exact time and the observer's task at that moment.
- Schedule control observations at a different circadian time where feasible.
- Avoid comparing night and daytime sessions unless equipment, staffing and environmental differences are documented.
- Stop work when participants become unsafe or too fatigued to follow procedures.
What circadian science cannot establish
A circadian rhythm cannot explain every event reported at night. It can, however, identify a predictable source of variation in attention, mood, reaction time and sleep propensity. The presence of a biological mechanism does not prove it caused a specific experience. That requires evidence linking the mechanism to the event.
Conclusion
The biological clock influences when people feel alert, sleepy and ready to perform demanding tasks. Night-time research should therefore treat time of day as an experimental variable, not merely an atmospheric choice. Documenting circadian context can improve safety and reduce avoidable bias without dismissing witness experiences.
AERM Core Principle: No anomaly is considered paranormal until environmental, psychological, technical, and scientific explanations have been systematically evaluated.
References and further reading
- Czeisler, C. A. and Gooley, J. J. “Sleep and circadian rhythms in humans.” Cold Spring Harbor Symposia on Quantitative Biology.
- Duffy, J. F. and Czeisler, C. A. “Effect of light on human circadian physiology.” Sleep Medicine Clinics.
- National Institute of General Medical Sciences, educational resources on circadian rhythms.
- American Academy of Sleep Medicine, resources on circadian rhythm sleep-wake disorders.
- Roenneberg, T. Internal Time: Chronotypes, Social Jet Lag, and Why You're So Tired. Harvard University Press.
About AERM
AERM develops investigation procedures that account for human factors alongside environmental and technical measurements. Safety and methodological transparency take priority over dramatic scheduling.
Educational and safety disclaimer
This article is educational and does not diagnose sleep or circadian disorders. Do not drive or operate equipment when sleepy. Consult a healthcare professional for persistent sleep problems.