In her early 40s, Ada Calhoun found herself racked with doubt about past and future decisions. Despite a long marriage and professional success as a journalist and author of the book “Wedding Toasts I’ll Never Give,” she lay awake at night worrying about credit card debt and evaporating job opportunities. She had heavy periods and crying jags. There was a sketchy mammogram and she was gaining weight. Her response to her image on her smartphone, even after putting on makeup and changing filters: “Still house of horrors.”

“I knew I felt lousy, but I didn’t yet fully understand why,” she writes in the introduction to her new book “Why We Can’t Sleep: Women’s New Midlife Crisis.” As the title suggests, the book is her attempt to answer that question.

Actually, it’s pretty clear from the introduction why Calhoun was feeling lousy. The real mystery: Were other Gen X women — those born between 1965 and 1980 — brooding over this same constellation of problems? And what cultural, economic and biological factors might undergird the malaise?

Calhoun interviewed hundreds of middle-class American women and plumbed the literature of midlife, from Gail Sheehy’s classic “Passages” to Darcey Steinke’s 2019 “Flash Count Diary.” She spoke to gynecologists, economists, sociologists and a “small army of therapists” to contextualize what she learned.

The resulting book is a sprint through everything — and I mean everything — that is bothering Generation X women, from irritation with slacker husbands and endless nagging email threads about school bake sales to fears of ending up in a cardboard box on the street. It’s a remarkably slender and breezy book, given the sheer quantity and variety of existential dread Calhoun has managed to funnel into its pages. If you aren’t having trouble sleeping already, you may start to after you’ve read a few chapters.

The anecdotal evidence Calhoun marshals for widespread Gen X unhappiness is abundant and depressing, if not scientific. Using only their first names and sometimes no names at all, Calhoun introduces us to a blur of agitated women in their 40s, most of whom we meet for a paragraph or two. There’s a woman who regularly pays a babysitter so that she can go to a movie theater and cry. A fed-up single mother smashes her son’s iPad with a hammer. A Silicon Valley executive worries she’ll lose her edge if people see her and think “Oh look, here comes the dowdy middle-aged woman.”An unmarried aerobics instructor muses, as she loads her boombox and gear into the car every week, “Will I never have a man to help me?

Everyone in this book wonders whether she’d be happier if she’d chosen a different path. Everyone feels slightly — sometime desperately — disappointed in herself and in her life, no matter how it might look to an outsider.

Why is this? Calhoun offers a plethora of explanations. To start with, there was the divorce and instability of many 1970s and 1980s childhoods, which made Gen Xers fearful of both marriage and ending a marriage, even a bad one. Calhoun calls up invidious role models of the era whom many of us “still keep in our psychic filing cabinet,” such as the foxy briefcase-carrying, bacon-frying blond woman in the 1980 Enjoli perfume ad who suggested women could, with no apparent effort, do it all. A lot of Gen Xers entered the workforce when the job market was soft, and now they find themselves sandwiched between two more populous demographic cohorts — the boomers, who haven’t retired, and cute, energetic millennials, who sometimes seem to have an edge with employers.

Gen Xers (along with a few older millennials) — are also the last group who remember the world before the it went online and, Calhoun argues, we “have no natural immunity to the Internet.” Gen Xers spend more time on social media than boomers or millennials and are vulnerable to its toxic effects. As Calhoun writes: “In our mothers’ and grandmothers’ eras, phones and mirrors sensibly sat on tables or hung on walls. But Generation X women noticed their first wrinkles in a zoomable smartphone camera.”

Throw in some hormonal mayhem, and it’s no wonder the women in the book feel blue. The trouble is, there are far too many of them. Reading “Why We Can’t Sleep” is like attending a party where the hostess didn’t want to leave anyone off the list: It’s noisy, crowded and everyone remains a stranger. And they’re all complaining. There are guests whose complaints we would benefit from hearing more about and others who shouldn’t be here at all.

In the book’s last chapters, Calhoun unveils her personal remedies for at least some midlife woes: She began hanging out with female friends, treating her perimenopausal symptoms with hormone therapy, and spending less time on social media. She stopped being so hard on herself. She touts the value of laughter. The advice is common-sensical, a little corny and hardly a panacea for the multitude of problems she’s spent the previous 200 pages describing.

But the final chapter is the most accessible and engaging in the book. Calhoun’s ambitious wide-angle shot of Gen X midlife malaise is blurry and overwhelming. Paradoxically, when she zeroes in on a specific woman with a first and last name, a strong voice, and a textured backstory — herself — that larger picture starts to come into focus.

Jennifer Reese is the author of “Make the Bread, Buy the Butter.”

Why We Can't Sleep: Women's New Midlife Crisis

By Ada Calhoun.

Grove/Atlantic. 288 pp. $26.99