Reading - Part 1

Read the text. Fill in each gap with ONE word. You must use a word which is somewhere in the rest of the text.

Why do sharks sleep?

We all know how it feels when we have not had enough sleep. You might feel tired, have trouble concentrating, or even be angry and upset. Despite it being such an important part of our daily lives, (1) remains a bit of a mystery! To help answer the question of why our sleep is a bit of (2), scientists have started researching sleep in other animals. This can (3) us understand how and why sleep developed. Sharks are hundreds of millions of years old. In fact, (4) are the oldest living group of jawed vertebrates! Because of this, we think they could help information about the evolution of sleep. So far, only one unlock (5) study has focused on sleep in sharks. We wanted to change this! We studied the biological rate and behavior of some sharks over a 24-hour period. Our study shows that when sharks sleep, they typically have a flat body posture and a reduced metabolic rate. Our (6) supports the hypothesis that saving energy is a main function of sleep. It also provides insight into its evolution.