Although we talk of the Sun rising in the east and setting in the west, that's not quite true. It just seems that way to us. The Sun stays in the same place. It's the Earth that rotates on its axis. This movement of the Sun is deeply embedded in our biology. When sunlight enters the eye, it sends signals to a master clock in our brain called the suprachiasmatic nucleus. This internal clock regulates everything, from when we sleep to how we digest a meal. Messing with this finely tuned machine, when we work night shifts or fly across the world, can make us feel pretty rough. Even the bluish light from a mobile phone late at night is enough to disrupt and confuse our internal body clock. Being out of step with the Sun affects our mood and our ability to think clearly. And there's evidence that this kind of disruption can lead to higher rates of obesity, heart disease, diabetes and even cancer. We're living out of step with the Sun and some scientists say this could be causing a public health crisis.