People have long been told by health experts that they should get eight hours of unbroken sleep each night. Nevertheless, many struggle on a nightly basis to fall asleep and stay asleep. Some scientists have begun to reconsider this issue and have concluded that the health experts have not been giving very good advice. In a study conducted in 2015, subjects were kept in darkness for 14 hours each night, this being an attempt to create conditions similar to natural wintertime conditions. Deprived of artificial light, most subjects' nightly sleep time eventually expanded and divided into two roughly equal shifts with 1 to 3 hours of wakefulness between. The results of this study suggest that human sleep is not naturally concentrated in one block. In fact, having two 'sleeps' a night may be the natural human condition. Historical data also support these findings. Historians have uncovered documents dating back to ancient times that refer to a 'first sleep' and 'second sleep.' Rather than view waking in the middle of the night as a nuisance, people appear to have welcomed it as natural, useful, and even enjoyable. By the end of the 1700s, street lighting in cities became more common, people stayed up later, and writers mentioning first and second sleep less and less. Most people seem to have adapted to consolidated sleep, even if they do not get a full eight hours each night. However, researchers argue that some modern sleep disorders such as insomnia may be the result of enforcing an unnatural sleep rhythm. In other words, waking in the middle of the night may not indicate an inability to sleep, but rather an attempt by the brain to sleep 'naturally.'