It's essential to apply sunscreen every day. The sun's rays can still damage the skin when it's cloudy, and it does not discriminate based on skin tone.
You should aim to use tretinoin every night, however, you must start slowly as your skin needs to get used to it. The first 2 weeks apply it 2 to 3 nights per.
A second treatment 7-10 days after the first treatment is recommended to kill newly hatched head lice. Therefore, even when using over the counter lice killing.