Sadness

Every single time, anytime I stay up past around 11:00 at night, I start to get really sad for what seems like no reason. Sometimes I'll get a bit paranoid as well. My boyfriend always asks what's wrong, and when I say that I don't know he tells me it's gonna be okay and he lets me hug him but I still can't shake the sadness. And it gets worse when I have to go to sleep and he has to go away. Sometimes my mom notices, but she just doesn't believe me when I say I don't know what's wrong. She thinks I'm keeping something from her. But I WISH that I knew what was wrong! Do any of you ladies experience the nighttime sadness as well? Could it be just symptoms of where I'm still going through changes? I'm 16 years old, if that helps. Comment any of your experiences or anything that may help, please!