Polyethylene film is the most powerful plant-warming material. In addition, it retains moisture and allows you to make watering more rare. Therefore, under the spring sun, the soil under the film warms up well and the seeds sprout quickly.
But after this, the film must be immediately removed, since the seedlings overheat and die just as quickly. To avoid overheating, it is usually suggested to make holes in the film or to use a film that allows air to pass through, but in reality this means giving up some of the solar heat. And on the other hand, at night the bed with shoots under the film cools very quickly and is easily damaged by frost.
I suggest another, more economical way to protect against overheating. Namely: to store up the accumulated heat for the future. The easiest way to do this is with water and plastic bottles. Water has a large heat capacity, and plastic bottles are very convenient, and also easily accessible. We place the bottles filled with water on a bed under a film. In this case, the solar heat under the film will be distributed not only to small shoots, but also to bottled water, as a result, the temperature under the film will not reach values dangerous for shoots. At night, the heat stored in the water will maintain an elevated temperature under the film, which will save the plants from frost.
Bottles can be put, put, stuck in aisles or suspended on supports - depending on the size of shoots. I just want to warn against the temptation to use bottles filled with water as supports for the film, since the water conducts heat well and it will immediately evaporate. In most cases, it is more convenient to use empty bottles as supports, for example, stuck in the soil, while water-filled bottles are laid horizontally.