* Darkness: The primary reason is that stars, planets, galaxies, and other celestial objects are much easier to observe when there's no sunlight interfering. The Earth's atmosphere scatters sunlight, making it difficult to see faint objects during the day.

* Clearer skies: The atmosphere is often calmer at night, leading to clearer skies with less turbulence. Turbulence causes twinkling and distortion, making it harder to get sharp images.

* Less light pollution: Artificial light from cities and towns makes it harder to observe faint objects. Observing from remote locations with minimal light pollution is ideal.

* Instruments optimized for darkness: Telescopes and other astronomical instruments are designed to work best in low-light conditions.

While some astronomical observations can be done during the day (like observing the Sun), the majority of research requires nighttime observations.

It's important to note that astronomers don't always work at night. They also spend time analyzing data, developing theories, writing reports, and collaborating with other researchers, all of which can be done during the day.