Between activities, homework and friends, it can be hard for teenagers to get a good night's sleep on a consistent basis. A new, Boston-based study finds a lack of rest can have serious health implications later in life."Duration of sleep is one thing, but when you sleep is something that we haven't studied in the past," said Dr. Elsie Tavares, the division chief of general pediatrics at Massachusetts General Hospital.She's also senior investigator of a recent study published in JAMA Pediatrics, focused on the timing of teenagers' sleep and what it could mean for their health.Researchers studied kids ages 12 to 17, asking them if they were "night owls," having an evening preference or "morning larks," preferring to go to bed and wake up early."Those adolescence that had an evening preference and an evening chronotype had a much higher risk of having obesity, having adiposity and having higher waist circumferences," Tavares said. The study found the effect impacted girls more than boys. Tavares and her colleagues suspect the results could have something to do with appetite hormones."When those hormones are set off track because we're either up too late or up too early, those appetite hormones can work against us," Tavares said.A consistent sleep schedule is key for teenagers. They should be getting between eight and 10 hours a night and the focus should be quality sleep.Here are a few tips: Start with a reasonable bedtime. It can be hard with after-school and evening activities, as well as homework, but have a daily bedtime goal in mind. Limit caffeinated drinks before teens go to sleep. Also, try to keep device screens out of their bedrooms. Tavares said making these changes now could set healthy patterns for life."Sleep is somewhat under-recognized as a risk factor for poor cardiovascular health and obesity and is just as important as healthy nutrition and healthy physical activity," Tavares said.The study also looked at the impact of social jet lag: the difference in how much teens sleep in on a weekend versus during the school week. Social jet lag also puts teens at risk for higher waist circumference and obesity.