I feel drained after my first hour on the mat on medium heat level. Is it ok?

It is not normal to feel dehydrated. 

You should not feel drained or have any discomfort and if there is some, please, restart decreasing the temperature, length, and frequency of the procedures to the lowest possible.

You may have the following things done incorrectly:

• the too high heat level for the beginning
• a too long time of the procedure for the beginning
• not enough rehydration/drinking of water and minerals 

In general, hot stone mats should be used on much lower temperature settings than standard electric heating devices, and they demand extra care, especially for all-night use of a midsize or larger mat.

The heat is activated when you lay on the mat, and it penetrates into the body. Unless you lay on the mat, It will feel only warm, even on the highest settings, and not too hot to the touch, even though the heat sensor might indicate that it’s high. This allows the FIR mats to generate deep penetrative heat yet not burn the body as conventional pads might.

You should choose the length, the intensity, and the frequency of the procedures that make you feel most comfortable and refreshed.

Start using the mat on the lowest settings, 30-40°C, and only for 15-20 minutes. 

Then increase the temperature by several degrees and 5 minutes each other day, and only if you feel comfortable.

And, again, drink a lot of water, mineral water, or herbal tea, and replenish minerals like calcium, potassium, sodium, magnesium, and others that your body may lose.

Be sure to have the doctor's approval. It is essential if you have any health condition. And read the owner's guide thoroughly.