Apparently
Research has [...] demonstrated that ‘pleasure readers’ do more bedtime reading which promotes peaceful sleep at night (Ponniah & Priya, 2014) and reduces stress and anxiety, as the pleasurable activity of reading helps secrete more serotonin. Serotonin is a neurotransmitter that works synergistically with melatonin, the sleep-inducing hormone, to give contentment and peaceful sleep (Buchanan, 2007). (Ponniah 116)
Admittedly there's no mention here of what happens to readers who are so engrossed they stay up late and are then exhausted in the morning when they can't sleep in, but it sounds like a great explanation/excuse for why bed-time reading should be encouraged.
So, here's a very short story by R. Joseph Ponniah about the benefits of late-night romance reading.
Once upon a time, a well-educated woman "approached the researcher asking for assistance and suggestions for improving her telephone conversation skills and language used in business contexts" (118). Her first language was Tamil, and she wanted to improve her English for use at work.
First, the researcher instructed that she watch a series of videos designed to teach business English. However, the videos were too boring, and the woman said so.
Then the researcher suggested that she read a series of novels about war, written by male authors. However, the woman said that these books were not of interest to her.
At last, the researcher thought to ask the woman about the novels she enjoyed reading in Tamil, and asked her to find similar books in English and
She collected more than 50 novels published by Mills and Boon and Harlequin and started reading them one after another without any assistance from the experimenter. In six months she completed 27 novels in [...] English [...], including Song of the wave (Anne Hampson), Scandalous (Charlotte Lamb), Temporary bride (Patricia Wilson), Kiss the moonlight (Barbara Cartland) and Married in a moment (Jessica Steele). She read at bedtime as she had more work during the day, and now she continues to read novels in [English] for pleasure. Discussion with the subject confirmed that reading such novels is a pleasurable experience and this kind of reading gave her not only the confidence to speak on the telephone but also helped her clearly communicate the intended meaning both in written and spoken language. When asked about her L2 reading experience, she said that she read for pleasure and did not feel that she was reading in a second language. [...]
She used to seek the experimenter’s assistance to draft business letters, but once she had been reading English novels for pleasure for about a month she developed the ability to draft without the support of the experimenter. She explained that she never experienced writing apprehensions after reading the novels in L2. Further, her confidence level increased when telephoning in English as she had acquired the skills required for conversation. (119)
And that is the story of how a little bed-time romance reading helped one reader gain proficiency and confidence in a second language!
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Ponniah, R. Joseph. "First-Language Reading Promotes Second-Language Reading and Acquisition: Towards a Biolinguistic Approach". The Idea and Practice of Reading. Eds. R. Joseph Ponniah and Sathyaraj Venkatesan. Singapore: Springer, 2018. 113-124.