What is a second sleep? Well it turns out that we didn’t always sleep right through the night. For most of human existence (until the industrial revolution), we did not have control over lighting the way we do with electricity. After sundown and dinner and campfires and what not, humans would then go to bed, waking up after 4 hours, then stay up for two to three hours, before going back to sleep for another four hours. This theory has been hypothesized and popularized by historian Roger Ekirch, and more recently a study was done by Thomas Wehr.
They had eight healthy men stay confined to a room for fourteen hours of darkness every day for a month. In the beginning, participants slept for about eleven hours (likely making up for previously lost sleep). After this adjustment period, the subjects in the study began to sleep just as much as people in pre-industrial times had. They would sleep for around 4 hours, wake up for 2-3 hours, then return to bed for an additional 4 hours, this second period of sleep becoming known as the second sleep. This is not the only alternative theory and method of sleep, but it is an interesting one to keep in your mind as you try to optimize your sleep.