“A study led by Yadan Li at Southwest University in Chongqing found that exposure to frightening images and sounds at night (20:00) produced greater increases in skin conductance, heart rate, and blood pressure than the same exposure during the day (08:00), regardless of room lighting conditions.”

A study led by Yadan Li, Wenjuan Ma, Qin Kang, and others, published in the International Journal of Psychophysiology in 2015, investigated nighttime fear. The study collected physiological data, specifically skin conductance response and heart rate, along with psychological self-reports. It manipulated time (day vs. night), illumination (light vs. darkness), and stimulus type (fearful vs. neutral) using visual and auditory stimuli. The results indicated significant increases in fear responses at night, with the difference between day and night being significant for fearful stimuli but not for neutral events, and these effects were consistent across different sensory modalities. The study did not explicitly mention measuring blood pressure or the specific times 20:00 and 08:00, but it did manipulate illumination conditions separately from day/night timing.

Arousal, defined as a state of physiological alertness resulting in increased attention and cortical activity, along with changes in motivation and emotional state, is a key factor influencing cognitive performance. Light can modulate arousal directly or indirectly via the circadian modulation of arousal, and the circadian clock regulates the ascending arousal system and the autonomic nervous system.

This study investigated physiological responses to fear-inducing stimuli, finding that skin conductance level (SCL), skin conductance response (SCR), heart rate (HR), and respiratory rate (RR) increased significantly. It also notes that previous studies have demonstrated emotional stimuli can elicit spontaneous reactions from the autonomic nervous system, affecting various physiological signals such as heart rate, respiratory rate, blood pressure, skin conductance, and body temperature.

Subjects who exhibited elevated depressive and anxiety symptoms had higher tonic EDA, skin temperature, and heart rate, despite not engaging in greater physical activity, compared to those that were not depressed or anxious. Most strikingly, differences in EDA between those with high versus low symptoms were most prominent during the early morning. The plots illustrate that SCL is elevated, particularly in the depressed group and most prominently in the early morning/night time hours. This is supported when modeling the diurnal patterns using a cosinor model and also in an OLS model testing tonic SCL and controlling for demographics.

Research indicates that the recall of conditioned fear extinction exhibits a circadian rhythm in humans and rodents, with optimal extinction recall occurring during the early active phase. This rhythm depends on the time of day of extinction recall, rather than the time of day of extinction learning, suggesting a rhythm in mechanisms supporting extinction memory maintenance and/or retrieval.

Circadian rhythms have been documented for various physiological variables, such as body temperature, heart rate, blood pressure, cortisol, adrenaline (epinephrine) and noradrenaline (norepinephrine) under resting, as well as exercise conditions. Body temperature follows a circadian rhythm in which temperature is highest in the evening and lowest in the early morning. Exercise heart rate follows a similar rhythm, but peaks a little earlier. The findings for exercise blood pressure are not consistent.

Electrodermal activity, such as skin conductance level (SCL) and non-specific fluctuations, is often used to reflect anxiety anticipating future threat, while heart rate may reflect fear of current threat. Some studies have reported greater skin conductance activation in panic disorder patients compared to control groups under various conditions.

This study explores the bidirectional relationship between sleep duration and clock gene expression, noting that sleep, a core circadian rhythm, maintains physiological homeostasis. It suggests that poor sleep impairs positive emotions and enhances negative emotion susceptibility, potentially involving circadian clock genes and neuroinflammatory pathways.

This study investigated how a daytime nap affected the consolidation of fear learning. It found that a wake group showed significantly larger skin conductance responses to conditioned fear stimuli compared to a sleep group, which showed no such difference. This suggests that sleep can influence fear generalization, offering a different perspective on time-related effects on fear memory.

Fear memory in species varies according to the time of the day. Although the underlying molecular mechanisms have been extensively explored, they remain largely unknown. Here, we report that hippocampal Rac1 activity undergoes a time of day-dependent alteration both in nocturnal rats and diurnal tree shrews and that training at the lower hippocampal Rac1 activation period during the night leads to better contextual fear memory in rats. However, other studies have shown that mice acquire contextual fear memory better during the inactive day, and diurnal humans display increased fear responses at inactive night (Chaudhury and Colwell, 2002; Li et al., 2015).

Research on psychophysiological responses to fear stimuli often measures heart rate and skin conductance. One study by Globisch et al. (1999) continuously recorded electrodermal activity, heart rate activity, and blood pressure during the presentation of fear-relevant and fear-irrelevant pictures, finding that fear-relevant pictures initiated a sympathetically dominated autonomic response in high-fear subjects.

Studies assessing physiological reactions to social rejection have shown that subjects who were socially rejected exhibited increased heart rates. However, social rejection had no effect on subjects' skin conductance levels in this particular study, although both conditions showed heightened arousal on this measurement.

Research suggests that terrifying film music can mimic alarming acoustic features of human screams, such as roughness, which could potentially signal danger through both music and the voice. This indicates how auditory stimuli can evoke fear and arousal.

The circadian rhythm regulates the brain's sleep/wake cycle, determining perceived peak alertness and responsiveness to external stimuli. Disruptions to rhythm can alter these responses, necessitating objective psychophysiological measures. We found that mismatched chronotypes exhibited significantly larger dark-enhanced startle than matched chronotypes. Chronotype mismatch can elevate the risk of exaggerated reactions in occupations that disrupt rhythm, such as air traffic controllers or emergency department physicians.

Dim light at night (dLAN) disrupts circadian organization and influences adult behavior. Exposure to dLAN at either age increased anxiety-like responses in adults. In contrast to adults, dLAN exposure during early life increases anxiety and fear behavior.

The results indicate that nighttime dim light exposure can cause functional changes of the circadian system, and suggest that altered circadian function could be one of the mechanisms underlying the adverse effects of light pollution on wild life ecology and human physiology.