My niece has been experiencing moments where she blacks out for a second or two every now and then. It happens really randomly and even during her sleep. She almost collapses for a brief moment and then regains control. She has no idea when it'll happen next, and we're really worried about it. What kind of specialist should we see for this? Could this be a sign of something more serious in the future?

Your niece's symptoms sound like they could be related to fainting or syncope, which is a brief loss of consciousness caused by a temporary decrease in blood flow to the brain. Given the random nature of the blackouts, it's essential to consult a specialist to rule out any underlying conditions. I'd recommend visiting a pediatric cardiologist, as they specialize in heart problems in children, which could be a potential cause. Additionally, a pediatric neurologist might also be helpful in case the issue is related to the nervous system. While most cases of fainting are harmless, it's always better to be safe than sorry, especially since your niece is experiencing these episodes randomly and even during sleep.