Jet-lag and the circadian rhythms
We might not notice our biological clock working day to day, but if you've ever been jet lagged then you'll realize how important it is to keep this clock in check! Normally humans sleep at night and are active during the daytime. Variables associated with activity and food intake, such as core temperature, blood pressure, plasma adrenaline and urinary excretion, are higher in the daytime than at night, whereas those associated with recuperation (plasma growth hormone and cortisol, for example) are higher at night. But if you've ever flown across time zones for business or holidays, you may be familiar with the uncomfortable, exhausting experience known as jet-lag. The fatigue, lack of concentration, and interrupted sleep at night mean poor physical and mental performance and a lot more errors. Some people also experience loss of appetite and indigestion or upset bowels. All of these effects are caused by multiple, complex processes, but a key player is the 'body clock' \cite{10656004}.