Note that NYC (or any town) might have requirements on top of what the state law requires.
The state laws happen to be fairly similar.
Reflector required only at night? I thought all the time. I'm in NY. If only at night, then of course I use lights.
Yes, only at night. (Note that bicycles are required by federal law to be
sold with reflectors.)
Here's a typical example.
http://www.dot.state.pa.us/bike/web/bikelaws.htm
Section 3507. Lamps or other equipment on pedalcycles.
(a) Lamps and reflectors. -- Every pedalcycle when in use between sunset and sunrise shall be equipped on the front with a lamp which emits a beam of white light intended to illuminate the pedalcycle operator's path and visible from a distance of at least 500 feet to the front, a red reflector facing to the rear which shall be visible at least 500 feet to the rear, and an amber reflector on each side.
This law is continued in an
atypical manner (most states, as far as I have read don't include this kind of stuff).
Operators of pedalcycles may supplement the required front lamp with a white flashing lamp, light-emitting diode or similar device to enhance their visibility to other traffic and with a lamp emitting a red flashing lamp, light emitting diode or similar device visible from a distance of 500 feet to the rear. A lamp or lamps worn by the operator of a pedalcycle shall comply with the requirements of this subsection if the lamp or lamps can be seen at the distances specified.