Seeing flashes on hearing loud or sudden sounds

(It can also happen on being touched unexpectedly or feeling pain)

(This isn’t considered actual synesthesia but rather a temporary cross-sensory phenomenon that can happen to anyone)

Some people report that when lying in bed with their eyes closed, about to fall asleep, if they hear a loud or sudden noise they sometimes see a flash of white light, a bright colour or a pattern of lines or stripes. There can be different colours or patterns, but the most common visual reaction seems to be a white flash. This visual reaction to sound isn’t considered synesthesia, as it can affect synesthetes and non-synesthetes alike and is actually fairly common.

 

What are the possible causes of this phenomenon?

There are several possible explanations for it, and the causes may be different for different people or at different times.

One likely reason is that it could be pressure generated by an ocular contraction reflex on being surprised by the loud or sudden sound. The muscles in your eyes and face tense suddenly, giving rise to a sudden ocular movement that briefly changes the pressure on the retina or optic nerve, and this creates a phosphene effect similar to the one you might see if you close your eyes and press your fingers lightly against your eyelids.

These phosphene patterns can also sometimes be generated by a blow to the head (“seeing stars”), a sharp pain or even, for some people, being unexpectedly touched by someone.

Another possible explanation is a startle reflex combined with a brief cortical discharge. The brain has an automatic alarm and orientation system that responds to sudden sounds, and an unexpected noise may activate areas of the brain that include the sensory cortices.

Alternatively, it could be due to a micro-awakening and consequently an abrupt brain-state transition. While falling asleep, the brain rapidly shifts between wakefulness, light sleep and partial awareness, and a sudden sound can briefly reactivate alertness. According to some theories, such flashes could be our subjective perception of this abrupt transition or “partial reset” in brain activity.

In the state between sleep and wakefulness we are prone to a phenomenon known as hypnagogic hallucinations. These are simply natural sensory perceptions, and they are usually visual: flashes, geometric patterns, and then, as the state deepens, fleeting dream-like images of people and objects. When this process is underway, if it is interrupted by a sudden sound you might suddenly become much more conscious of this dream-state activity and experience it for a moment as a waking perception.

 

These are some of the main theories, although others exist. In any case, although this phenomenon consists of a momentary crossover between senses, it is different from the kind of perceptions that a person with auditory-visual synesthesia, chromesthesia etc. would experience. These are consistent over time and are recurring fixed associations rather than random apparitions, so listening to the same sound, musical note, chord, timbre or even song or musical genre produces the same visual impression each time. Also, a synesthete’s visualisations (“photisms”) are usually perceived in the mind’s eye or just “felt” as a strong impression, rather than being physically seen as in the case of nocturnal flashes (although a minority of synesthetes do literally see their colour and shape impressions as if they were projected in the air in front of them).

This is what Sean Day says about the subject in his book Synesthetes (2016/2021):

"There are indications that people in general – that is, regardless of whether they are congenital synesthetes or not – have hypnagogic synesthetic experiences at least two or three times per year. Such experiences are usually ‘sound à visual’; most commonly, a loud noise, such as a door slamming, produces a synesthetic visual flash. There are other possibilities for hypnagogic synesthetic experiences, though, such as a touch to the body making you think someone is calling your name. Research is currently underway on this topic1."
1 See, for example, Sagiv & Ben-Tal (forthcoming).

 

There are also people who are especially sensitive to what is called the “sound-induced flash illusion”, which has been the subject of scientific experiments: on watching a single flash of light and hearing two beeps, they see two consecutive flashes.

Here is some information on the sound-induced flash illusion and a test to see whether it happens to you.

More information on entoptic phenomena (visual effects originating within the eye itself) and other related topics:

Closed-eye hallucinations

Hypnagogia (sleep hallucinations: hypnagogic hallucinations occur in the profoundly relaxed state of transition from wakefulness to sleep, while hypnopompic hallucinations happen at the moment of waking). This excellent article also discusses the science of hypnagogia and other aspects. (Note: hypnagogic hallucinations occur quite normally from time to time for many people and don't necessarily indicate any kind of sleep disorder).

Two kinds of visions, synesthesia and hypnagogia: a comparison. An article by artist Carol Steen comparing synesthetic and entoptic/hypnagogic visions.

Go to the page on auditory-visual synesthesia

Readers’ comments - this page contains the comments on this article received between 2022 and 2025, with readers’ own experiences and some alternative possible explanations for why it happens

This page last updated: 15 May 2026