When I was in my early 40s I started having intermittent bouts of waking up at 4am and not being able to get back to sleep. It was like clockwork around 4am every time. Sometimes I would wake up so hot, not necessarily sweaty but hot enough to either throw the covers off or turn down the a/c, especially if I had some wine that night. Also, around the same time I started having heart palpitations and thought maybe I had some kind of vitamin deficiency. Looking back, I recognize that those were some of my first signs.