The claim that the Guardian's article is making is that people who get less sleep consistently are more likely to die prematurely, and this is true. The claim is based off of research done by the University of Warwick and Federico II Medical School. After extensive research, researchers at the Universities found that people who consistently sleep for less than six hours a night were 12% more likely to die prematurely than those who consistently sleep 6-8 hours. In addition, the researchers found that people who consistently sleep for longer than nine hours have an increased chance of dying prematurely. Sleep is incredibly important as it is linked to our physical and mental health, and consistently sleeping poorly (both undersleeping and oversleeping) can cause short term issues such as fatigue, mood, and concentration, and long term issues such as an impaired immune system, an increased chance of developing chronic illnesses, and premature death. Professor Francesco Cappuccio of the University of Warwick said, "Modern society has seen a gradual reduction in the average amount of sleep people take, and this pattern is more common amongst full-time workers, suggesting that it may be due to societal pressures for longer working hours and more shift-work. On the other hand, the deterioration of our health status is often accompanied by an extension of our sleeping time." He suggests that there are societal factors that cause a lack of proper sleep, and subsequentially, premature deaths.
The study that the claim was based off of is reliable. The study was performed by two well known universities and well qualified people. Moreover, there were around 1.3 million participants, with follow ups for up to 25 years after the study was performed, which proves the validity of the data.
Based on these factors, we can deduce that this claim is true. It was given by a reliable source, and many other articles and journalists agree with the data analysis performed by the researchers at the universities.
https://warwick.ac.uk/newsandevents/pressreleases/short_sleep_increases/
https://www.mentalhealth.org.uk/explore-mental-health/publications/sleep-matters-impact-sleep-health-and-wellbeing#:~:text=Sleep%20is%20an%20essential%20and,brains%2C%20not%20just%20our%20bodies.