Although the garment known as a nightcap may have fallen largely out of vogue, at one time it served several important functions for its wearer. A nightcap was generally worn along with a loose-fitting but substantial nightgown, padded slippers, and possibly pajama pants. If the wearer needed to rise from the bed during the night, a full-length robe may also be wrapped and tied around the entire sleepwear ensemble. The nightcap itself may be a snugly-fitted short hat or a larger conical cap.
In the days before reliable central heating and cooling systems, most homes were heated by wood burning fireplaces or coal-burning furnaces. This form of radiant heat was notoriously inefficient and difficult to maintain throughout the night. Unless a bed chamber was equipped with its own fireplace and a servant to keep it stoked overnight, there was every chance the fireplace would stop providing heat at some point. This made proper sleepwear and bed coverings essential. Since much of a person's body heat escaped through his or her head, a insulating nightcap could make a significant difference in the comfort level of its wearer.
Many nightcaps depicted in period literature and paintings were noticeably longer than standard caps and hats. While this may have given nightcaps a somewhat comical appearance, there were legitimate reasons for their exaggerated length. A nightcap often took the place of a daily wig, which in itself could be substantial in size. The nightcap protected the wearer's natural hairstyle from becoming too unkempt in the morning. The additional air space in the nightcap also trapped warm air and kept it closer to the wearer's head.
As bedrooms became more regularly heated and cooled, the need for nightcaps as insulating sleepwear eventually waned. Some people continue to wear sleep hats for other reasons, however. Certain hair treatments and styles require the protection provided by some form of head covering at night. A good nightcap can also reduce the severity of "bed head," a familiar condition in which the sleeper's hair becomes completely unmanageable in the morning. Others may choose to wear a nightcap for the same insulating benefits experienced by generations before them.