Circadian rhythms cause your body temperature to drop just a couple degrees when you are going to sleep. Then your body temp goes slightly high, in the time just before you wake. Because of that, the increased temperature difference between your body and the ambient tricks your body into thinking you are hot. Of course that is assuming you keep your room warm. Sweating during sleep could also be a fear response to nightmares, even if you don't remember them.
it's because you having and bad dream so that when your heart bet at an fast beat and you began to sweat more in your sleep .or you have Hyperhidrosis i know this because i have this same thing with my feet and hand it okay
..When the Body Perceives that it has Ingested or been Exposed to a Chemical or Poisen or Dangerous Medication...It will sometimes Drench the Pillow with Fluids/Water...Trying to get Rid off the offending Substances.Whatever the Cause in your case...it is Best for you to Drink Plenty of Water and Fluids to help you get all balanced-out again.Westie.