People who spend sleepless nights know how it adversely affects both body and mind to function the following day. Sleep is meant to be a regenerative process for the body, much like recharging an electronic gadget. In the absence of sleep, both mind and body tend to lose their vitality. A person may end up feeling fatigued and find his/her thoughts to be clouded.
In short, one may find oneself incapable of physical work and having clarity of mind. Other effects can include headache, dark circles, irritability, indigestion, and so on. There can be various reasons for this sleep disruption, the major one being melatonin.