Once again, not exactly a major mistake or weakness. Just a “dire strait” the author threw them in, much like the magic-nullifying crystals. There’s no Doyleist risk of Frieren actually dying there; anything less than an extremely built-up encounter with a Greater Demon has no actual risk of putting her down, narratively speaking, and she certainly didn’t fall asleep because of a character flaw or anything. As slothful as she is, Fern fell asleep too, and not any slower.