Choose the correct alternative (when - while).

1. I had a bad dream while I was sleeping on the sofa. 2. Karina was doing her homework when the lights went out. 3. I was talking to my husband in the park when two cars crashed behind us. 4. You called me last night while I was studying for an important exam. 5. My mother found some old photographs while she was cleaning her room. 6. The students were doing an exercise when the bell rang.