In the far reaches of my memory, I can picture a scene from my childhood. My little brother and I are in his bedroom, playing with our Playmobil figurines. We put the children to bed, as we were always put to bed. But the adults? We announce, with some glee, that they are going to stay up all night.

Oh, the irony. My adult reality is a long way from wanting to stay up all night because I can. As the mother of five, I feel like I spend copious energy some days trying to convince everyone else in my house to fall asleep so I can too. During what I call the “busy years,” when people are building a career and raising a family, sleep can feel like a fair-weather friend — elusive, even, if you’re craving quality time.

Yet it turns out there’s a paradox here. While many of us feel frequently exhausted (The National Sleep Foundation reports that adults feel sleepy, on average, three days a week), reputable time diary studies show that even busy people get adequate sleep from a quantitative perspective. When I did a time diary study a few years ago of 1001 days in the lives of women who had six-figure jobs and who had kids at home, they turned out to average 7.7 hours of sleep per day. Even CEOs of large companies get about 7 hours per day.

So why do we feel so tired?

The problem is that the averages are hiding a big problem. For many people, sleep is disorderly: short some nights and unexpectedly (or desperately) long others.

This is obvious enough for people with babies, or who work unpredictable shifts, but it’s widespread. In my study of women with six-figure jobs and kids, I found that 22% slept at least 90 minutes more or less on Tuesday than they did on Wednesday.

Undershooting or overshooting on sleep can lead to fatigue on some days and schedule chaos on others — with widespread impacts on mood and productivity.

You can see how this could play out in your life. You sleep in Sunday morning and have trouble falling asleep Sunday night. But you have to be up early on Monday, so you start the week getting less sleep than your body needs. Monday is full — if not frantic — and you stay up late again and get up early Tuesday. The sleep debt accumulates.

But there’s no debt forgiveness when it comes to sleep, and soon your body forces you to pay. You crash on the couch while watching TV on Tuesday night, or you fall asleep while putting a kid to bed on Wednesday. Thursday morning you hit snooze or sleep through your alarm. Saturday morning you sleep in or nap, but then you’re up late Saturday, sleep in on Sunday, and the cycle starts again.

It’s like one of those drop-tower carnival rides, yanking the hapless passengers up and down.

That might be fun at an amusement park, but it’s no way to sleep. It’s far better to get the amount of sleep you need each night — consistently. Since most adults need to wake at set times for work or family responsibilities, the only variable that can move is the time people go to sleep the night before.

In other words, you need a bedtime. You need to go to bed on time, at a set time, unless you have a really good reason not to. When you get your ideal amount of sleep every single night, instead of skimping and crashing, you’ll have more energy for everything else.

How do you go about choosing a bedtime? It’s a simple math problem.

  1. Figure out what time you need to wake up most days. For instance, I need to get up at 6:30 AM during the week to get my teenager to school.
  2. Count back by the amount of sleep you need. I need about seven and a half hours, which is a good number to start with if you’re not sure. This gives me a bedtime of 11 PM. Someone who needs to wake up at 5 AM and who needs seven hours of sleep would have a bedtime of 10 PM. Someone who needs eight hours of sleep and who doesn’t need to be up until 8 AM could have a bedtime of midnight.
  3. Set an alarm for 30 minutes before lights out. Use this time to wind down.

It sounds simple, but it’s life changing.

As part of the research for my latest book, I asked 150 people to observe a regular bedtime for nine weeks. Before the project and at the end, I asked people whether they were getting enough sleep to feel well rested. The number of participants who agreed that they were getting enough sleep rose 25% from the beginning of the program to the end. Scores rose 13% on the question of whether people had enough energy to handle their responsibilities. As one participant noted, “Getting enough sleep helped me be my best self both at work and afterward. I have had enough energy to get to everything I have planned this week, and that makes me happy.”

Now, to be sure, life can torpedo even the most well-intentioned bedtime. A distressing work email, a kid’s panic over an undone assignment, or a wailing baby can disrupt everything. And, to be honest, we sometimes keep ourselves up. After the chores are done and the house is quiet, we can putter and scroll. As one person said, “That’s the only truly free time I have.”

I, too, often wish the minutes before 11 PM would move slower. But if you find yourself tempted to stay up late to score more leisure you can remind yourself of two things.

First, we are adults. If there is a good reason to stay up later, go for it. Like those Playmobil figures, you can stay up all night! Setting a bedtime just nudges a decision so sleep deprivation doesn’t happen mindlessly.

Second, we can build me-time into our lives at other points too. I recommend taking one night off from work and family responsibilities each week, and carving out conscious breaks to read or do other hobbies.

Then you can sleep and have your fun too. But that only works if you give yourself a bedtime. Perhaps that sounds like no fun, but I’ve come to see that going to bed early is how grown-ups sleep in. Put that way, a bedtime sounds almost like a treat.