I was rereading the PH, and I noticed this. In the section about Darkvision, it says:

Darkvision

Many creatures in fantasy gaming worlds, especially those that dwell underground, have darkvision. Within a specified range, a creature with darkvision can see in darkness as if the darkness were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned. However, the creature can't discern color in darkness, only shades of gray.

I then referenced the condition lightly obscured, which reads:

In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.

 So I was wondering, who actually plays with this ruling? I know it is a rather niche condition, since many parties will need a light source anyway with some non-Darkvision players. But I am DMing a campaign with all Darkvision possessing, somewhat nocturnal PCs.

Do you enforce this in your campaign? 

I would enforce it at my table, but admittedly it is easy to forget in the moment. If everyone has Darkvision, that makes the bookkeeping much easier.

Quote from UndauntedDM >>

Do you enforce this in your campaign? 

 I do in my current game, though next game I'll be strongly tempted to bring back 3.5e-style night vision so almost everyone needs a light source.

I encountered some of the niche conditions with dark vision and specifically shadow stealth discussed in this thread https://www.dndbeyond.com/forums/dungeons-dragons-discussion/rules-game-mechanics/120631-shadow-stealth-encounter

But yeah my players were just playing in ravenloft so of course they have disadvantage... ;) 

In my campaigns, Darkvision see normally in dimlight and lightly obscured in darkness, which grant disadvantage on Wisdom (Perception) checks that rely on sight.

I've always thought it strange that many campaigns have underground cities where all the denizens have darkvision and so there is no lighting.

Darkvision is very short range for normal living (usually 60 ft) and grey-scale, so having normal lighting would be greatly beneficial to seeing further, and seeing colours - as well as making it easier to spot intruders!

Lighting helps Darkvision users to see better, farther and in color so i see most sophisticated underground cities relying on light source. Creatures living in complete darkness usually to do because they lack the ressources to steadily run light either from torch, oil lantern or magic.

I use it.

In my games, most inhabited underground areas of races with darkvision will at least be dimly lit. There may be darkness in tunnels and connecting passages since the inhabitants don't need light to easily navigate these areas but inhabited areas will be dimly lit. However, it is pointless for these folks to brightly light their living spaces since dim light is treated as bright light for creatures with darkvision so there is no point in using the fuel or resources needed to keep their areas brightly lit.

I've had under dwelling creatures that use light also use coloured paint for signs which can't be easily discerned with darkvision.

For the poll, both answers are true:

They don't need a light source; but they will have disadvantage without one.

The underlying problem is that the DM wants the party to succeed.  Most of the time, you want someone in the party to notice the thing.

Quote from MusicScout >>

The underlying problem is that the DM wants the party to succeed.  Most of the time, you want someone in the party to notice the thing.

 Not necessarily. If they don't find the thing = maybe it is a secret door and they thus have to take the long/dangerous way round.

Otherwise there's no point in making the party roll to notice something, if you as DM are always going to want/have them succeed.

Quote from MusicScout >>

The underlying problem is that the DM wants the party to succeed.  Most of the time, you want someone in the party to notice the thing.

 Depends. If the thing is "secret door if not found the plot stops in its tracks", yeah, I want the party to notice it. If the thing is a trap or an ambushing monster or some side loot, I'm fine with the party missing it.

In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.

Characters with darkvision that also have a light source have a choice whether to use it or not.

Different DMs can have individual values such as wanting characters to succeed, wanting to give them an eventful storyline and/or wanting them to earn success.  In each case, the DM could stick with RAW but decide whether to throw in additional bits of help such as the occasional insight check such as on the merits/dangers of lighting up.

I forget this all the time