Are there any long term cognitive impairments from prolonged period reduced sleep? What are the consequences of chronic lack of sleep? For example, insomniacs are known to sleep less than normal sleeping individuals. 

I am wondering if there are any long term cognitive impairments from reducing sleep time to, say 4h instead of the normally recommended 8 hours in the long run, say for a year or so?
Wikipedia has some good information on this issue. Search for:

Effects of sleep deprivation on cognitive performance

In summary, 'yes', sleep deprivation can have a multitude of negative consequences on your cognitive abilities, like weakening your ability to handle complex cognitive tasks.