You already know about the importance of sleep. Neuroscientists have confirmed what Arianna Huffington has been preaching for years: Getting enough sleep is vital to your productivity, health, effectiveness as a leader, and long-term brain function. But how much is enough? More than you might think, according to psychology PhD, Penn State adjunct professor, and sleep researcher Dan Gartenberg

For most of us, eight hours of sleep a night is the holy grail of good sleep practices: Often aspired to, not always achieved. And yet, in a fascinating Q&A with Quartz, Gartenberg explains why you should probably be getting at least half an hour more--and how you might be able to improve your sleep habits and fit more shut-eye into your daily routine. 

Here are some highlights:

1. Even if you feel well-rested and alert, you probably need more sleep. 

"When you're sleep deprived, research has shown that you're really bad at being able to tell that you're sleep deprived," Gartenberg told Quartz deputy ideas editor Georgia Frances King. If you really want to know how much sleep you should be getting he proposes a simple (and pleasant) test: Go on a vacation completely away from the distractions of work where you can sleep as late as you like. Go to bed at your usual time, and then see what time you naturally wake up. After a few days, he says, "you'll just fall into a natural pattern, and that's probably how much sleep you actually need."

2. Eight and a half hours is the new eight hours.

Gartenberg says that, according to one of his Penn State colleagues, eight and a half hours should be considered "the new eight hours." Why? Because even people without chronic insomnia spend about 10 percent of our time in bed not sleeping--we're either falling asleep or slowly waking up. "If you're in bed for eight hours, a healthy sleeper might actually sleep for only about 7.2 hours," he explains. That's why, in order to get eight hours of actual sleep, which is what many people need, normal sleepers need eight and a half hours between when they close their eyes and when the alarm goes off.

3. You can make your time in bed count with good sleep practices.

The better your sleep hygiene, the more quickly you'll fall asleep and the better you'll sleep, both of which will give you the maximum benefits of whatever amount of time you spend in bed. According to Gartenberg, ideal conditions include a cool temperature (if you and your partner aren't comfortable at the same temperature it's a great idea for each of you to have your own bed covering or even a heating pad on one side of the bed); silence; and darkness, with blackout shades if your bedroom has light pouring in the window at night. You will also give yourself a better night's sleep if you use your bedroom only for sleeping and sexual activity (i.e., don't work in bed or have an office in the bedroom if you can avoid it). And avoid blue light, which means light from electronic screens such as a television, smartphone, computer, or tablet right before bed.

4. Naps are a great way to get the additional sleep you need.

Gartenberg has one piece of good news: You may need eight and a half hours of sleep but you don't have to get them all at once. If you wake up for a bit in the middle of the night, that's OK--in fact it's how our ancestors slept in the days before electric lighting was common. And it's perfectly fine to get some of your sleep time in the form of an afternoon nap, a time when many people feel sleepy. 

"Maybe I'll get a little less [than eight hours] during the night, and then I'll take a 20-to-30-minute power nap at midday," he says. "There's a siesta for a reason!" Many workaholics try to get through the whole day without much of a break using coffee or other stimulants, he says. But giving in to that late afternoon sleepiness and grabbing a nap might boost your productivity more. "We weren't made to produce for eight hours straight."