House temperature???

I live with just my dad. he keeps the house thermostat at 55 and supplements with a wood fire, but the house's overall temperature is usually not more than 60. the rooms furthest from the fireplace are below that since the central rarely comes on while the fire is burning. we are not poor, it just seems that the cold doesn't both him at all and he likes conserving energy. is this too cold? is it safe to remain in such temperatures for extended periods of time?