There's a good reason why Shakespeare called sleep "chief nourisher of life's feast". It is as important as food for physical and mental health.Of course you have to make sacrifices sometimes in order to finish your PhD, but sleep should be the last thing you sacrifice.Cutting down your sleep even by a small amount per night can have serious negative effects;

• Decresed alertness and cognitive performance
• Impaired memory
• Irritability and stress

To succeed at PhD level research you need your brain working to the best of its ability. So giving up sleep might gain you an extra ahour or two of working time, but if you can't think straight then that extra time isn't very useful.If you deprive yourself of sleep consistently over a long time it also affects your immune system meaning you are more likely to become ill. So any working time you gained by cutting back on sleep, you can lose in sick days.Sometimes research demands a late night. There were times when I was still in the lab when the sun came up. But it's not sustainable in the long term and you have to give yourself time to recover!If you have too much work, if you are stressed and not making fast enough progress, slow down and think about how you work. Depriving yourself of sleep is never the answer.