According to the passage, what problem does the recent research find?

(18)New research shows that a lack of sleep is a growing health problem around the world. Lack of sleep is not just a problem in developed nations. It’s getting just as bad in developing countries as well. Researchers at the University of Warwick Medical School in Coventry, England conducted the study. The purpose was to look at the existing data from eight different countries from both Africa and Asia. Researchers came to estimate the prevalence of self-reported sleep problems across eight different populations. The research was conducted in Ghana, Tanzania, South Africa, India, Bangladesh, Vietnam, Indonesia and an urban area of Kenya. The study estimates 150 million adults in developing countries are suffering from sleep-related problems.(19)It shows that there is biological evidence supporting the notion that sleep deprivation, for example, may impair important physiological functions. And lack of sleep also has an impact on the immune system, which may actually explain the association of sleep with occurrence of many diseases. Sleep problems are associated with unhealthy habits as well, such as smoking and a poor diet. Some people can actually sleep too much, such as the elderly, making them more prone to disease. The study recommends that sleep patterns be included in assessing a population’s overall health.(20)It also says lifestyle changes should be considered before prescribing medication.