By side effects it would depends if you are talking about the long term or the short one. Short one is quite obvious, it will be that you will feel sleepy and find it difficult to concentrate. Long one would be the damage done to your body. Some may argue that they survived fine with a mere 3-4 hours sleep only. That also depends on the activity you have carried out during the day. First day of heavy work and 4 hours sleep might be enough for you to wake up refreshed tomorrow due to your own mental motivation. However, after the second day with the same amount of work and the same amount of sleeping time, you will find yourself harder to wake up on the third day. Constantly pushing your body to its limit will undoubtedly do some good amount of damage to yourself.