Tomatoes are a warm season vegetable that can be grown in a variety of climates.
However, tomato plants can only handle full sun for a limited period. When it is too hot for tomatoes to handle, cover them with a canopy or row cover to keep them cool and protect them from damage.
There are two main reasons you would want to cover tomato plants at night:
Prevent heat damage and extend the growing season by protecting the plants from frost. While nighttime temperatures in the summer do not generally get below freezing, overnight lows during winter months sometimes dip into the 20s and 30s. Covering your tomatoes with a shade cloth will help to warm them up while they are still cool enough to sleep.
If the temperature ever dips below 50 degrees Fahrenheit, remove the shade cloth and give your tomato plants some extra protection by covering them at night.