As a purely bedtime garment, the nightshirt is the successor of an undergarment used by Europeans through the 19th Century. This undergarment was worn all day under outer clothing. At night the outer clothing was removed, and men and women slept in the long shirt-like garment. This smock was longer for women than for men. Upon arising in the morning, the outer garments were again put on.

The major difference in the use of the nightshirt is that it replaces all clothing worn during the day. It is removed after arising and put away for use during the next sleeping period.

From Wikipedia