Cats average about 15 hours a day sleeping, sometimes as much as 20! This is because they are crepuscular creatures which means they are primarily active at dawn and at dusk as this is the best time for them to be hunting their prey.
In terms of sleeping less than they need and side effects - well since cats are so good at short naps and dozing it's rare for them to have sustained periods of lacking sleep - if something has kept them awake more than they would like they can quickly and easily "catch up". When experiencing prolonged sleep deprivation the health of the cat is affected negatively (like all animals basically) and there's evidence of psychological effects as well. There's an interesting paper on these effects here
Not really sure on sleeping too much as this would usually be a symptom of an underlying issue (either medical or mental) so it would be hard to say what this would do in isolation other than the obvious things like weight gain and loss of muscle strength.
Is sleeping pattern of feral cats the same as indoor house cats? Is there any different form of sleeping pattern in indoor house cats due to absence of natural events of a forest?
Indoor cats are likely to sleep slightly more than a predominantly outdoor cat due to the abundance of safe and comfy places to sleep and the easy availability of food. They still sleep in broadly similar patterns though - even an indoor cat that has never even seen a predator still does much of their sleeping in a "dozing" state that allows for quick responses to things around them.