Not so!Current(U.S.)regulations state the following:
AT NIGHT--All vessels at anchor must display anchor lights. If your vessel is less than 164feet/50 meters in length, then its anchor light is an all-around white light visible where it can best be seen from all directions.(ie.mast head)
EXCEPTIONS--If your vessel is less than 23 feet/7meters in length, then it is not required to display an anchor light or shape (day) unless it is anchored in or near a narrow channel, fairway or anchorage, or where other vessels normally navigate.
If your vessel is less than 65.6 feet/20 meters in length, then it is not required to display an anchor light if it is anchored in Inland Waters in a special anchorage designated by the Secretary of Transportation. (Prudence would dictate however, that you always show an anchor light when anchored at night, regardless of the area)
Larger vessels have their own requirements.