Here are complete sentences reflecting on the questions:

1. **What does physical exercise do in the brain?**
Physical exercise enhances brain function by increasing blood flow, promoting the growth of new neurons, and improving cognitive abilities such as memory and learning.

2. **How much time do you need to get enough sleep?**
Most adults typically need between 7 to 9 hours of sleep per night to function optimally.

3. **Will getting enough sleep help?**
Yes, getting enough sleep can significantly improve overall health, cognitive function, and emotional well-being.

4. **Why do young people suffer from insomnia?**
Young people often suffer from insomnia due to factors such as academic pressure, increased screen time, irregular sleep schedules, and stress.