Many countries apply different forms of nighttime restrictions during pandemic: curfews, early restaurant closings. What is the reasoning behind that? Intuitively it seems to me that people are less likely to crowd up during nighttime.
The spread of the pandemic can be reduced by reducing contacts between infected and uninfected people. Since many infected are unaware of their infection, the goal is to reduce contacts between people in general. (Few countries have enough testing capacity to reliably identify the infected part of their population.)
The maximum reduction of contacts would be to order everyone to stay at home for a couple of weeks. But the side effects of such an order would be severe. So the goal of many governments is to reduce unnecessary contacts while they maintain necessary contacts at the minimum level to avoid the worst side effects.
What are necessary contacts?
That's a highly controversial question in most countries. Medical services for sure. The food supply, certainly. There are doctors who argue that careful outdoor sports is better for the overall public health than ordering people not to do sports, so perhaps solo jogging or walking the dog. The manufacturing industry, to supply shops which have to stay open and for the economy. Schools, perhaps, because remote digital education doesn't work as well as face-to-face contacts, and also because closing schools greatly affects parents in vital sectors of the economy. For other businesses, it becomes a tradeoff between their infection risk and economic importance.
(When Germany reopened after the first not-quite-lockdown, car dealers were rather early. They move a lot of money per contact.)
What are unnecessary contacts?
Again a highly controversial question in most countries. Unfortunately for the restaurant industry, they are rated as less vital than supermarkets and manufacturing, so they get closed. Same for theaters, cinemas, nail studios, ...
(As a contributing factor, many people go out to a restaurant or bar to get drunk, and as they get drunk their hygiene discipline suffers. They get closer to each other. They touch. That's really bad for the sober restaurant-goers, but they're caught by the blanket curfew.)
Lockdown measures are working with a very broad brush:
daytime = work =
good less bad, nighttime = leisure = bad.
Events like parties, clubbing, concerts, etc happen past curfew. These events have the potential to infect a lot of people. The problem with such superspreader events isn't just the number of people they infect but also that they also serve as additional bridges between different social circles.
Another way to look at this questions is this: Why are there night-time restrictions, instead of closing leisure-time facilities like restaurants and clubs outright?
And the cynical answer to that question is that it's done this way because it helps with shifting the brunt of the crisis onto affected businesses and their employees, instead of paying for it from tax money or letting landlords and banks take the loss:
- If you close restaurants completely, the owners and employees have a strong legal and moral argument to get compensation from the state. In that scenario, the immediate reason why their business is in trouble is because of a law. For rented business properties, it's also possible renters would not be required to pay rent anymore, because the rental contract may say something to the effect that if the property is not usable for the purposes specified in the contract (through no fault of the renter), then rent doesn't need to be paid. Then the landlords might not be able to afford their mortgage payments, which would lead to losses for the banks and large investors. Those latter have huge political clout to prevent this in the first place, or to sue the government afterwards. No matter where the actual loss is pushed to in the chain (businesses, landloards, banks), someone would have reason to sue the government or at least have very strong political arguments to demand money.
- On the other hand, if you just restrict opening times and plead with people not to go to those businesses, then you can argue that there are always (in most countries) restrictions on opening hours and that if people choose not to go to a business, that is their own private decision. So in a sense it's just business as usual and obviously the state is not required to pay money to businesses which are in trouble just because of normal economic turmoil. Similarly, there is pretty much no way to sue landlords for rent reductions.
At a simple level (and a number of governments are jumping for simple solutions), prior to Coronavirus, most social contacts happened in the evenings - after work. People got together, visit each other's houses, go to a pub or restaurant, nightclub or movie theater, concerts etc. These are "non-essential" situations that are more likely to result in person to person contact inside, particularly as the weather gets colder in the Northern Hemisphere. As o.m. points out, there was also concern that any situation where alcohol or other intoxicants are involved mean people become less concerned about keeping distance, keeping masks on etc. So by introducing a curfew, the government is trying to help enforce the division between "necessary" work or food-shopping trips outside and "for fun" visits that have more risks attached.
There appear to be multiple factors at work here.
The main difference clearly visible is a different attitude towards work and leisure. Anything that is not a productive industry or necessary for survival has been hit hard by the lockdowns in almost every country. That includes art, entertainment, all kinds of leisure industries, travel (where most countries have strict rules on tourism and make exceptions for business travel) and bars/restaurants.
Some of this has a rational basis. While you may be exposed to the same number of people at work than in a restaurant or bar, these are also the same people every day, while in a restaurant/bar it would be different people every time you go there. So the number of unique contacts over a week or month is significantly higher.
But a large factor is also that these industries are considered non-essential. Our culture has a relation to leisure activities that is some or more than some puritanical, i.e. leisure is seen as guilty pleasures. Work is a necessity, leisure is a luxury. This cultural factor makes it easy to close down leisure industries, and the political fallout is much lower than if you closed down anything that's "actual work", despite that fact that lots and lots of jobs that are open are bullshit jobs.
This is most clearly visible in travel. In many European countries right now, hotels are not allowed to give rooms to tourists, but they can rent to business travellers. Anyone who has spent any time in business knows that most business travel is bullshit and could be replaced with an online meeting or even dropped entirely. The reason for most business travel is that a) face-to-face meetings are more personal, making many deals and discussions easier and b) people like to travel and if the company payss, so much the better.
So the reason for night-time lockdowns is that during the day, "proper" work would be seriously affected by any restrictions, while at night only the guily pleasures of leisure are stopped. Our work/day schedules largely adhere to this principle of the work day and leisure evening, and there are exceptions for the jobs that don't (24/7 activities like hospitals and police). Work has lots of lobbyists and industry groups on its side. Leisure much less so.
Most establishments are daytime establishments anyway, and the few who have nighttime opening hours and are not essential tend to be ordered to shut during curfew as well. So with the exception of essential services such as hospitals (which are exempt from the curfew if you need to visit one), nothing is open at night.
People who are out at night are out predominantly to socialize with one another. That's the exact behavior that is being restricted by the lockdown measures, hence the restrictions.
Additionally, there is less enforcement at night. This is going to incentivize those who would otherwise socialize during the day to try and do it under the cover of night, to avoid the scrutiny that the pandemic restrictions have brought with them.
An additional reason to enact a curfew / night-time restrictions to fight the pandemic is that it can indirectly help curtail contacts between people in a private setting. This type of night-time restrictions can complement bans on public events, bar closures or other restrictions on hospitality businesses while still allowing other activities (office and factory work, retail) to go on.
Some countries have forbidden private contacts outright or strongly recommend reducing them (rule of six, support bubble, maximum of two guests, etc.) but others have deemed such restrictions unconstitutional or impossible to enforce and rely on curfews to make meeting friends or organizing parties much more difficult while stopping short of a stronger “lockdown”.
Of course, guests can still spend the night and a gathering during the day is not inherently safer but it's a game of numbers: most people will find it more difficult to meet in those conditions, the average number of contacts will be lower and that's the desired effect. On the other hand, I am not sure I can name any country that has sustained night-time restrictions for a long time or where they have been enough to reduce case load. In France for example, it feels more like a last-ditch effort to manage the pandemic that was quickly replaced by even more restrictive rules.
Part of the reasoning (not mentioned in other answers yet):
Sunshine is a good disinfectant. source
Mornings and noons are, in general, times to work. Aternoons and nights are for leisure times. Usually, during working hours people are demanded to be sober; during leisure times there is no obligation (except for anecdotal "Don't drink and drive.").
Night times are mostly partying, visitting pubs, concerts and other venues where:
- many people gather at small and poorly ventilated rooms. This increases the transmission probability.
- people from very wide range of circles do meet there. This increases the spread of the infection.
- As people are less and less sober they are less and less careful and less and less responsible. Just google for "Hold my beer..." tropes. Which boosts the transmission probability near certainity.
Plus in many countries smoking in bars is forbidden so all the smokers get from the heated overcrowded places outside for "the fresh air" where they gather in tight pack so they don't get that cold.
Considering that the lower reproduction number (how many new cases on average one patient causes) the easier to hold the pandemy "within bounds" it is. If the number is near one, it is critical, if it is significantly higher it is uncontrollabe explosion, if it is below one, the situation is stabilizing.
Estimating such reproduction number for a night venue or pub session I would bet for a value of N-1, where N is a total ammount of people inside.
TL;DR: One may think of closing night events as removing the gas pressure bottles from the building which is on fire. The house is still on fire, but it won't explode soon.