It is unclear what exactly causes hot flashes but at menopause women don't produce enough of the hormones that tell the brain to regulate temperature. The low estrogen level confuses the body's thermostat & sends a message to the body's organs to work harder which brings on a sudden sensation of heat. The body then cools down which gives you a chill. This can disrupt your sleep and make it difficult to get enough rest that is needed to revitalize the body.
Something that can you can do is to dress in layers so when a hot flash comes on a layer can be removed. Once the flash is over the layer can be used to cover up when the chill comes. Also at night, spread a few towels between you and the sheets. After a hot flash, remove the first towel leaving a nice dry towel to rest on.