When you're on a deadline or have a particularly thorny project in front of you, it can be tempting to try to power through for long periods of time. But your brain's ability to focus only lasts about 90 minutes before you need to take a break. If you ignore signs from your body that you need a rest — difficulty concentrating, physical restlessness, irritability — your productivity will wane. Instead, completely focus your energy in 90-minute cycles. Set an alarm, turn off your phone, close unnecessary windows on your computer, and put a sign on your office door. When the time is up, take a needed break, even if you're on a roll — go for a walk, take care of an easy task, read an article, etc. Once rested, get back into your focused state.
Source: Harvard Business Publishing : Today's Management Tip was adapted from the Guide to Getting the Right Work Done.