The following article was sourced from a Wikipedia page at the following address: http://en.wikipedia.org/wiki/Nightshirt
A nightshirt is a garment intended for wear while sleeping. It is somewhat longer than most regular shirts, reaching down to the thighs or below the knees,[ leaving some of the legs uncovered. It is generally loose-fitting to avoid restricting the wearer's movement while sleeping.
Until the 16th century men slept naked or in a day-shirt; subsequently a night-shirt, varying in quality, was worn in bed. Nobles in the 16th century wore embroidered shirts or "wrought night-shirts". By the 19th century the night-shirt resembled a day-shirt with a loose, turned-down collar, or a loose, ankle-length nightgown was worn.
Occasionally worn in the 20th and 21st centuries by women as well.
To read more about nightshirts, please click on the following link: http://en.wikipedia.org/wiki/Nightshirt