Water has a specific heat capacity of about 4.2 Kilojoules per kilogram per degree Kelvin, so it would take about 6 Kilowatt Hours of electricity to heat 100 Litres of water from 15 degrees centigrade to 65 degrees centigrade. As a standard immersion heater is 3KW, so it would take 2 hours to heat 100 Litres of water.
Drinking a large amount of carbonated drinks at bedtime can also cause hiccups, as the air in these drinks can distend the stomach. Consuming a large amount of alcohol before going to bed is another possible cause of hiccups during sleep. The excessive amount of alcohol can cause the stomach to distend.
Hiccups are caused by a sudden contraction of your diaphragm. The diaphragm is a muscle under your lungs that helps you breathe in. The top of your windpipe (your glottis) closes immediately after your diaphragm contracts which makes the typical 'hic' sound. A hiccup is an automatic action of the body that you can't control (a reflex).