Our perception of time can be altered, especially in our modern world. A good example is trans-Atlantic flights, which force our brain to rapidly adjust to a new and often opposite schedule. We usually call this jet lag. This desynchronization between external and internal time can cause a series of symptoms like drowsiness during the day and insomnia at night. It may also cause headaches, loss of appetite, and irritability.

People subjected to this desynchronization can have a number of health issues, including metabolic diseases such as diabetes and insulin resistance. In a dramatic example, a series of experiments show that the hour of chemotherapy administration in cancer treatment can dramatically alter the response and toxicity of cisplatin.

To further complicate this time-induced variability, our endogenous rhythms vary from one person to the next; scientists usually split people in two categories, “larks” or early risers and “night owls.” These two groups are radically different. The former rise near dawn, are most productive early in the morning, and go to bed early, while the ones of us that fall in the second category are most productive late at night, go to sleep late, and also wake up late. The category where we belong is determined in part by our genes.