Hot water first thing in the morning wakes the body up by dilating the channels, it flushes the kidneys and facilitates evacuation of the mahasrota (gastrointestinal tract) by stimulating peristaltis. By dilating the channels hot water helps the body flush out impurities (ama).
It is recommended not to microwave the water. Best is to boil the water in an open saucepan for 5 to 10 minutes and so it is infused with pitta to get you going in the morning. Or water from the kettle will do.
For maximum benefit drinks should be sipped while your sitting down. Hot water to start the day is a profound inexpensive medicine that improves the metabolism.