For example, horror movies appear to be scarier when viewed at night than during broad day light.

Does light have any role in this phenomenon?

Are there changes in hormones at night versus during the day that makes fear stronger?

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    $\begingroup$ I think this is a really awesome question, I might try to edit it a bit if you don't mind, I'll try not to change the meaning. I might also write up a quick answer based on rodents, though something you may or may not realize is that "fear of the dark" is certainly not universal - if you were a mouse asking the question, you might wonder why the daytime is so scary. $\endgroup$ – Bryan Krause Jan 27 '17 at 22:24
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    $\begingroup$ I don't have time right now for a full response, but I'll just dump a couple starters out there. The hormonal side I'll have to do a little research on, there are certainly hormones that change throughout the day but I don't have detailed knowledge of this specific effect. There is also a direct projection from certain cells of the retina directly to the amygdala, an important brain structure in fear. The topic is actually of high interest because similar circuitry is implicated in anxiety disorders and PTSD and belongs in the broader category of "what makes 'normally' not-scary things scary." $\endgroup$ – Bryan Krause Jan 27 '17 at 22:36
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    $\begingroup$ @Bryan Krause: Indeed, fear of the dark is not universal even among humans. I've always liked the night myself, and can't really understand why people use nightlights & such. Nor have I ever noticed any change in intensity of 'fearful' stimuli that wouldn't be explained by not being able to clearly see the source. E.g. that growl might be a bear, or your neighbor's friendly dog. In the day, you would know (unless it's in the bushes) and react accordingly. In the night, you assume the worst case and prepare. $\endgroup$ – jamesqf Jan 28 '17 at 4:21
  • $\begingroup$ I think it would be necessary to report references that this is actually the case (fear response is activated more easily at night), otherwise it is just an opinion. $\endgroup$ – have fun Jan 30 '17 at 9:45
  • $\begingroup$ here: cogsci.stackexchange.com/questions/16751/… $\endgroup$ – can-ned_food Feb 21 '17 at 9:28

Short answer
The increased fear responses during the night are believed to be mediated by elevated corticotropin-releasing hormone (CRH) levels in the brain that drive the fear responses in the amygdala.

Fear responses can be experimentally assessed by recording the startle reflex. For example, loud noises can evoke gross startle responses in rodents (whole-body startles) and more subtle responses in humans (eye blinks).

In nocturnal animals, it has been shown that startles are facilitated by bright light. In contrast, fear responses in humans, a diurnal species, have been shown to be stronger in the dark (Grillon et al., 2007).

In the rat, the light-enhanced startle is has been associated with the hypothalamic–pituitary–adrenal axis (Fig. 1). Specifically, light exposure has been shown fear mediated by corticotropin-releasing hormone (CRH) (Grillon et al., 2007).

Fig. 1. HPA axis. source: Wikipedia

The HPA-axis is affected by the circadian clock and therefore exhibits a pronounced daily rhythmicity. The HPA axis is activated by hypothalamic release of CRH, which stimulates the secretion of adrenocorticotropic hormone (ACH) from the anterior pituitary. ACH triggers the production of cortisol by the adrenal glands. Cortisol, in turn, provides negative feedback and shuts down HPA-axis activity by binding to corticosteroid receptors in various HPA-axis structures, e.g.the hypothalamus (Fig. 1). As a result, cortisol levels show a robust diurnal rhythm that runs in opposite phase with CRH. Cortisol in humans peaks at the time of awakening and drops to its lowest levels around bedtime, whereas the opposite is true for CRH (Miller & Gronfier, 2001).

The startle reflex shows a diurnal variation roughly opposite to the rhythm of corticosterone and in phase with CRH. Specifically, startle responses in nocturnal animals such as rats are highest during daytime (their passive phase) when CRH is peaking, and corticosterone is at its lowest. Conversely, startles are lowest during the night (their active phase) when CRH levels are lowest, and corticosterone levels are high. In man, a diurnal species, exactly the opposite pattern is observed. Hence, startle responses are thought to be endogenously modulated by the circadian system, most likely via effectors of the central pacemaker such as the HPA-axis (Miller & Gronfier, 2001).

In rats, administration of exogenous CRH enhances the startle reflex and blocking of the glucocorticoid receptors which produces a blockade of negative feedback, also increases startles. Conversely, blocking CRH receptors decreases the startle response, as does corticosterone administration (Miller & Gronfier, 2001).

CRH receptors are expressed at various brain centers, including the amygdala. The amygdala is a key structure in the limbic system, the emotional control center of the brain. The amygdala is specifically associated with anxiety and fear responses. Indeed, it is thought that CRH receptors in the amygdala are responsible for the fear-promoting actions of CRH (Grillon et al., 2015).

Hence, it is not darkness or light per se that mediate fear responses. Instead, it is the circadian control of the HPA axis that controls the daily variations in fear responses.

- Grillon et al., Biol Psych (2007); 62(10): 1183–6
- Grillon et al., Neuropsychopharmacol (2015); 40: 1064–71
- Miller & Gronfier, Psychophysiol (2006); 43(3): 297–301

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    $\begingroup$ @Mesentery - no rush :-) Apologies for the delay, but complex questions like this one need some careful preparation :-) It took me a while to gather the appropriate references and get the story right. Especially because most research has been done in nocturnal species the subject is quite complex $\endgroup$ – AliceD Feb 28 '17 at 22:16
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    $\begingroup$ Great answer. Of course this is almost certainly only part of the story - I would expect there could also be learned and instinctual associative learning related to darkness and fear - the strength of which no doubt displays a lot of situational and inter-individual variability. And great find on that Grillon paper - I was searching for something similar when I initially commented and wasn't immediately successful, finding mostly the rodent work in the light instead. $\endgroup$ – Bryan Krause Feb 28 '17 at 22:29

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