Why do we sleep so late?

According to a representative sample of Great Britain, 80% of the population aged 16–95 years go to sleep between 22:00 and 01:00 hours, which is well after dusk for most of the year (Groeger et al., 2004). It is, of course, the artificial light which is shining at us from ever brighter lamps, screens of TVs, computers and gadgets which makes it possible for us to see, read, shop and socialize and not get enough sleep (Czeisler, 2013). It is becoming increasingly clear that this artificial light is sufficiently bright to delay the rise of the circadian melatonin rhythm and it may be that it is only during the weekend, when many of us are sleeping even later, that we sleep in phase with our delayed biological clocks.