[sarasi]

The 28 degree thing is part of the Japanese government's "anti global warming" initiative- they recommend that all homes and businesses set their a/c no lower than 28 degrees in summer. I personally find this quite comfortable and anything lower than 28 to be too cold, but different people find different temps comfortable.

Your host mother is probably also concerned about her electricity bills- there is quite a difference in power consumption between 28 and 24 degrees, so you should probably respect her wishes here.

The "fan at night making you ill/killing you" thing however is a complete myth- tell her you are willing to take the chance, and that not sleeping will make you sicker.