Tony Schwartz in HBR Blog Network: 

So why is sleep one of the first things we’re willing to sacrifice as the demands in our lives keep rising? We continue to live by a remarkably durable myth: sleeping one hour less will give us one more hour of productivity. In reality, the research suggests that  even small amounts of sleep deprivation take a significant toll on our health, our mood, our cognitive capacity and our productivity.

Many of the effects we suffer are invisible. Insufficient sleep, for example, deeply impairs our ability to consolidate and stabilize learning that occurs during the waking day. In other words, it wreaks havoc on our memory.

So how much sleep do you need? When researchers put test subjects in environments without clocks or windows and ask them to sleep any time they feel tired, 95 percent sleep between seven and eight hours out of every 24. Another 2.5 percent sleep more than eight hours. That means just 2.5 percent of us require less than 7 hours of sleep a night to feel fully rested. That’s 1 out of every 40 people.