According to the neuroscientist Matthew Walker—in his 2017 book, “Why We Sleep”—insomnia, strictly defined, is a clinical disorder most commonly associated with an overactive sympathetic nervous system, and it is triggered, typically, by worry and anxiety. Insomniacs can write twee lists of their blessings until the cows come home, but their cortisol levels will still tend to look as if they’re gearing up to storm the Bastille. – The New Yorker