All night long.
Cities: Skylines remains one of the most surprising titles this year, supplanting SimCity as the premiere city-builder with a thorough feature set, well-optimized graphics, plenty of lot space for expansion, extensive mod support, and no always-online DRM shenanigans. It's somewhat ironic that it took an indie developer, namely the Finnish studio Colossal Order, to show Maxis and Electronic Arts how city simulations are supposed to be done. The modding community continues to shine alongside the game, and indeed some of the most popular mods introduces a day/night cycle. It's a point that Colossal Order has since taken with its first expansion, aptly named After Dark.
On face value, having a day/night cycle might not seem to add much apart from a cosmetic touch. And to be clear about what the "day/night cycle" means here, your city will experience daytime effects for about a three weeks to a month of in-game time before switching to about a month of nighttime effects. That's roughly eight to ten minutes each on the lowest speed setting. As the sun fades over the horizon, the lighting becomes suitably dark, the windows of nearly every building begin to light up, and streetlamps illuminate the roads.
The developers added a textural layer to each building solely for the lighting, a feature that surprisingly doesn't cause the game to slow down whatsoever. Your city will seem to twinkle like Christmas lights as night descends, while landmarks shine confidently across your city's skyline. It brings a new brilliance and perspective to your city's design.
Nighttime is thankfully far more than a visual effect here. Traffic is less congested as the majority of service vehicles and daytime workers no longer clog the streets. Commercial buildings, especially those in the new leisure and tourism district specializations (like beachfront properties and arcades), continue to attract customers, though they produce some noise pollution to the surrounding area (though you can turn on a policy that closes these buildings during nighttime). Thieves and criminals are more active as well, so it's important to raise the budget for police during this time period.
In fact, the addition of a separate nighttime budget contributes to city efficiency tremendously. You can assign garbage truck vehicles to be more active at night, reducing traffic in the daytime and taking advantage of the open streets at nighttime. To support the nightlife in your city, you can bolster the transportation budget for public transportation, including the new taxi stands. By controlling how you much you spend on each area of the city by moving the daytime and nighttime sliders in the budget, you can save hundreds of dollars.
That said, After Dark doesn't fix some of the issues with the base game. If you don't understand why commercial buildings aren't getting enough customers or enough educated workers, or why traffic seems to be piling up in certain areas of the city, you will still be at a loss (unless you visit forums and user tutorials). Traffic is a bit more manageable this time around, but it still feels vague at times. Laying down water pipes and clearing out abandoned buildings still feel like needless busywork. An undo button would also have reduced the repair work necessary after making a slight mistake with a mouse click here and there.
While After Dark isn't necessarily essential to enjoy the Cities: Skylines experience, it would be difficult to imagine the game without this expansion moving forward, especially at its humbly low price. Even during nighttime, it shows off the game's pleasant presentation and smooth graphics, while effectively expanding your ability to fine-tune most of your city's services. Cities: Skylines may have been a dark horse at first, but with After Dark it's now a formidable thoroughbred.