Complex sentence: Before a heat stroke occurs, people show signs of heat exhaustion such as dizziness, mental confusion, headaches, and weakness; if a heat stroke occurs when the person is asleep, symptoms may be harder to notice.
Split into simpler sentences: Before a heat stroke occurs people show signs of heat exhaustion such as dizziness, mental confusion, headaches, and weakness. However, if a heat stroke occurs when the person is asleep symptoms may be harder to notice.