Sleep is still a mystery, despite how critical it is for our health.
Most of us spend roughly one third of our lives sleeping, but you may need more or less than eight hours a night.
Here’s what sleep scientists and doctors say about how much time you should spend sleeping.
'Self-care'
"The reasons aren’t entirely clear, but it’s an essential thing that we all do," said Dr Rafael Pelayo, a sleep specialist at Stanford University.
"Something remarkable happens when you sleep. It’s the most natural form of self-care that we have".
Most of the population gets between seven to nine hours and that particular category has the lowest association with health problems, said Molly Atwood, a behavioural sleep medicine clinician at Johns Hopkins.
Once people get either fewer than six hours of sleep or more than nine hours on average, the risk of health problems inches up, Atwood said, but everybody is different.
When you’re trying to figure out how much sleep you need, it’s important to think about the quality of your sleep, Pelayo said, adding that you want to wake up feeling refreshed.
"If somebody tells me that they sleep many hours but they wake up tired, something is wrong," Pelayo said. "You shouldn’t leave your favorite restaurant feeling hungry".
How much sleep we need changes
The amount of sleep we need changes throughout our lives. Newborns need the most –somewhere between 14 to 17 hours.
"Definitely when we’re babies and children, because we are growing so rapidly, we do need a lot more sleep," Atwood said.
The National Sleep Foundation recommends most adults between the ages of 26 and 64 get between seven to nine hours of sleep.
Older adults can get slightly less and young adults between ages 16 and 25 can get slightly more.
Humans cycle through sleep stages roughly every 90 minutes. In the first portion of the night, Atwood said that more of the cycle is slow wave sleep, or deep sleep, which is essential to repairing and restoring the body. It’s also when "growth hormone" is released.
In the latter hours of the night, more of the sleep cycle is spent in rapid-eye movement sleep, or dream sleep, which is important for learning and memory consolidation, or the process in which short-term memory gets turned into long-term memory.
Kids get more "deep sleep," with about 50 per cent of the night in that realm, she said. That drops at adolescence, Atwood said, because our body doesn’t need the same kind of repair and restoration.
Something else interesting happens around puberty: gender-based differences in sleep start to crop up.
Do women need more sleep than men?
Research doesn’t show that women need more sleep, but women do get slightly more sleep on average than men, Atwood said.
It starts at a young age. Though they have the same sleep needs, teenage girls seem to get less sleep than teenage boys, Pelayo said. Additionally, teenage girls tend to complain of insomnia more frequently.
When women become first-time mothers, they often care for newborns throughout the night more frequently, which means less sleep, said Allison Harvey, a clinical psychologist and professor who studies sleep at UC Berkeley in the US.
Hormones may also impact women's sleep quantity and quality during pregnancy and menopause.
Experts say women may also need more sleep right before their menstrual cycle.
When to seek help for sleeping
You'll know if you’re not getting enough sleep if you're feeling grumpy, irritable and inattentive. Long-term, those minor symptoms can become serious problems – even deadly.
"If you’re not getting enough sleep or you have untreated insomnia or sleep apnea, your risk of depression increases," Atwood said.
"Your risk of cardiovascular issues like high blood pressure, risk of heart attack and stroke increases. Your immune system is compromised. You’re at greater risk for Alzheimer’s".
If you’re getting the recommended amount of sleep every night but still waking up feeling tired, you might consider going to your general practitioner.
They can rule out other health conditions that may affect your sleep, Atwood said. But if problems persist, seeking out a sleep specialist could be helpful.
