“Exploding Head Syndrome”?
The night has a thousand eyes...
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My youngest son has this.
The brain does not transition well into sleep mode. There is a rush of synaptic activity that causes a loud booming sound inside the head, sometimes accompanied by a flash of light. It’s not dangerous or painful but can be very scary to a young child.
The first time it happened my son came running in crying. Freaked me out until I spoke to the doctor.
It helps to have a regular and soothing routine prior to bed. Story time, prayers, soothing back rubs, etc. Also, nothing sugary after 6 PM.