Blanketing?

I have started blanketing my ponies and horses. It is getting down to the early 40's and upper 30's at night. I have an arabian pony that has never grown a heavy coat and I have owned her for nine years. Another TB mare that we own has been through more than 5 surgeries and nearly a year later is just gaining most of her weight back. Yet another one of my ponies is clipped. I have been blanketing them and leaving sheets on during the day when its supposed to be below 65. My mother and I have gotten into a rather big argument about me to quick to blanket, is she right in her reasoning? I am doubting myself now. Is it ok that I am putting medium weight blankets on at night and leaving sheets on chillier days?