What did the medieval woman wear to bed?
Generally it depended on the
circumstance of her being in bed. Was she there to sleep,
lure her mate into an evening of romantic activity, give
birth or was she unwell and convalescing?

Sleeping
It seems that the usual thing for a woman to do when sleeping, was
to sleep naked- naked except for some kind of head covering.
The latest medical thinking believed that much of the body's
heat was lost through the head and that for the sake
of one's health, the head should remain covered.
Most images show this. It might be a veil, head wrap,
coif or even hairnet.
The image at the right comes from stained glass window and shows
Tobias and Sara. They are both in bed, wearing nothing but a coifs,
or in her case, perhaps a wrapped veil. Their little dog snuggles
down on their bed, as we know some pets did.

Socialising
Wealthy medieval ladies often socialised in their bedrooms
or solars, and when this occurred, a certain richness of
dress would be retained. A lady might wish to remain warm on a very
cold winter's day, but that's no reason to be any less elegantly
dressed, especially if one was receiving visitors.
This
French miniature, at left, is from the 13th century, "Queen
Blanche of Castile With Attendants."
She herself is wearing a
heraldic gown and so is the smaller woman to her side. Both
have veils and coronets- a crown in the case of the queen.
The queen also wears a brooch,
something not necessary for bedwear, showing that she has dressed
in a manner suitable for receiving visitors.
There is a guest with a scroll
in his hand casually leaning on the bed, hinting at a certain amount
of familiarity with the queen. The musician at the foot of the bed
also wears a coronet, although it is likely the two smaller people
wearing coronets could be important guests. The similar heraldry
hints that there may be a family connection.