In both, the main female lead mistakes the male lead for being gay.

In both, the main lead mistakes the other for being gay. In Heartbeat Love, the female lead mistakes the male lead. In Down with Love, the male lead mistakes the female lead.

When I first saw the transferring to another hospital with a poor condition in D-Day I immediately remembered RDTK. Although it won't be the focus in D-Day, I can still see the resemblance especially how the lights get turned off in the hospital. The dramatic accidents are very similar to those in RDTK because they have a similar feel. D-Day is based on Disaster genre, so there is more of that there. And they both have light romance that develop slowly.

Maybe am still stuck with Moon Lovers, but you can see how both revolves around history and present with reincarnation of the main characters. If you love destined romance which repeats itself despite the passing time, then you definitely will love this.

if you liked how Ji Sung in Blood has exceptional skills as a doctor, you might be also interested in Doctor Stranger and how the main lead develop exceptional skills to identify illnesses and cure them, sometimes without having someone to help him or having the proper materials.

Both have interesting plot worth watching =D

Lying and deceiving others.. both also talk about NIS agents. They are both action too.

The Moon that Embraces the Sun is also a historical drama just like Night's Watchman. They both have fantasy as their main genre.
They also involve Shamans. The main lead actress are both shamans.
The main actor in both dramas is a royal. In TMTETS he is a king, while he is a prince in the Night's Watchman.

Now that I think about it Il Woo act in both but in the MTETS, he is the older prince, in NW he is the younger prince.
I just kept being reminded of TMTETS while watching this drama =D