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Parents around the world know the scene: a teenager who cannot get out of bed in the morning, yawns through breakfast and then stays awake until well after midnight. For years, this behaviour was blamed on laziness or too much time in front of a screen. Research over the past three decades, however, suggests that the real explanation lies largely in biology, and that teenagers are not simply choosing to stay up late.

During adolescence, the body's internal clock shifts. The hormone melatonin, which makes us feel sleepy, is released later in the evening than it is in younger children or adults, often by as much as two hours. As a result, a typical sixteen-year-old simply does not feel tired at ten o'clock, even if he or she has been up since early morning. This change usually begins around puberty and does not reverse until the early twenties.

The problem is that school timetables rarely take this into account. In many countries, lessons start at eight o'clock or earlier, which means that teenagers must get up when their bodies are, in effect, still in the middle of the night. Since most of them need between eight and ten hours of sleep, many end up sleeping only six or seven hours on school nights. Lying in at the weekend helps a little, but it cannot fully make up for a week of lost sleep, and it can shift the body clock even later.

The consequences of this regular sleep loss go well beyond feeling tired. Studies have linked it to poorer concentration and memory, lower exam results and a higher risk of anxiety and depression. Sleepy teenage drivers are also more likely to be involved in road accidents, particularly in the early morning. Some researchers have even compared the effect of serious sleep loss on reaction times to the effect of drinking alcohol.

Some schools have responded by moving the start of the day later. When one city moved the start of its high schools from 7.50 to 8.45, a study found that students slept about half an hour longer, felt less sleepy in class and earned slightly higher grades; at one school, attendance also improved. Critics point out that later starts can cause difficulties for parents' work schedules and for after-school sports, but supporters argue that these practical problems can be solved.

In the 1990s and early 2000s, overnight trains across Europe seemed to be disappearing. One route after another was cancelled as budget airlines offered flights between major cities for less than the price of a restaurant meal, and new high-speed lines made many daytime rail journeys much shorter. By the mid-2010s, some national rail companies had stopped running their own sleeper services altogether, arguing that they were unprofitable.

Over the past decade, though, the trend has reversed. New overnight routes have been launched between a number of European capitals, and some services that had been cut have been brought back. On the most popular routes, tickets for private sleeping compartments now sell out weeks in advance, particularly in summer. Some operators are now ordering new sleeper carriages simply to keep up with the number of passengers who want to travel overnight.

The main driver of this revival is concern about climate change. A journey by train produces only a fraction of the carbon emissions of the same journey by air, and a growing number of travellers, especially younger ones, say they are willing to spend more time travelling in order to reduce their environmental impact. Some companies have even introduced policies requiring staff to go by train for shorter business trips. For these travellers, a slower journey is a price worth paying for a lower carbon footprint.

Night trains also offer practical advantages that travellers increasingly appreciate. Passengers board in one city centre in the evening and arrive in another the next morning, saving both the cost of a hotel and the time spent getting to and from airports. Unlike on a plane, there are no strict limits on liquids or luggage, and passengers can move around freely during the journey.

Nevertheless, operators face serious obstacles. Sleeper carriages are expensive to build and can make only one journey per night, so they earn far less than daytime trains. Trains that cross borders must also deal with different electrical systems, safety rules and track charges in each country, which pushes ticket prices up. As a result, a sleeper ticket can cost several times more than a flight on the same route. Unless governments provide more support, critics warn, the revival may remain limited to a small number of profitable routes.