If it is not polluted, it is not toxic if it is not eaten. In general, polyethylene is also non-toxic and produces toxic gases only at high temperatures. You can sleep on it.
It should be noted that the foam boards produced by small workshops are not necessarily clean pure polystyrene. If the garbage is directly processed into cushions without good treatment after recycling, it will have an irritating smell and have a certain impact on the body.
It is recommended to buy products produced by regular merchants, and don't covet cheap and inferior products, which will endanger your health.
I hope it helps you.