staying warm, especially if you have a shaved head

Stay Warm

in sitting meditation

use blanket, keep knees warm
if you have a shaved head, use a beanie, blanket, cloth, etc. to stay warm.
protect the neck too especially in windy conditions. use scarf, blanket, etc. as necessary
be careful of wind! it can dissipate body heat and qi very quickly. if you get a sore neck, headache, don't open your windows all the way, or sit in a place that doesn't get such direct draft.

only windbreaking material blocks wind

if you like to meditate outside, you really need to understand this principle.
Even if you're bundled up with the warmest, thickest wool, if you don't have an outer layer with windbreaking material, cold wind cuts right through wool, cotton, fleece, it feels as if you were naked.

I feel sorry for monks and nuns not keeping their head  and extremities warm

Many of them, if they're meditating in public or where the public can see them, out of devotion to vinaya rules keep their bald heads exposed. If you don't keep warm, and don't protect yourself from wind, you're going to have physical health problems, arthritis, aches and pains when you get older.