If you’ve ever had a particularly terrible night of sleep, where you just didn’t get much rest, then chances are you’ve felt the mental lag the next day. No amount of coffee can help pull your brain out of that sleepless slump and much of the day is a waste. Well, it’s not surprising then that studies show a chronic lack of sleep is a good indicator of cognitive decline down the line. “A good night’s sleep is essential,” said Dr. Yasmin Ohlsson, a family and lifestyle medicine doctor and consultant, with a special interest in preventative medicine. She notes that eight hours is ideal for most people.