Sites vary, but most mention a median range of between 30-40% humidity.
40-50% Humidity
During extremely cold weather, your home loses humidity to the
outdoors and may drop as low as 5%. By comparison, typical humidity in
the Sahara Desert is about 25%! Generally, one does not want to have a
home humidity any lower than 30%. Optimal comfort is considered to be
achieved at 40-55% humidity. Nosebleeds also occur in hot dry climates
with low humidity, or when there is a change in the seasons.
Above 30%
Indoor relative humidities should be kept above 30% to reduce the
likelihood of the occupant’s nasal passages drying out,[7][8]
Humans can be comfortable within a wide range of humidities depending
on the temperature—from thirty to seventy percent[9]—but ideally
between 50%[10] and 60%.[11] Very low humidity can create discomfort,
respiratory problems, and aggravate allergies in some individuals. In
the winter, it is advisable to maintain relative humidity at 30
percent or above.[12] Extremely low (below 20%) relative humidities
may also cause eye irritation.[13]
WebMD
Keep the heat low [60°F (16°C) to 64°F (18°C)] in sleeping areas