There is little research published in scientific journals about conscious adoption of extreme sleep patterns.
In 2000 PureDoxyk proposed the Uberman schedule:
Every four hours you sleep 20 minutes.
It takes a lot of willpower to adopt a sleep pattern like Uberman that only allows 2 hours of sleep per day. People need a deep inner desire to adopt such a schedule or there will be no compliance with the experimental setup.
Most people who try to adopt the Uberman schedule fail very soon. There are some such as Steve Pavlina or Tynan claim to have followed the schedule and produced a detailed account of the experience at their website.
Steve Pavlina quit the schedule after 5 1/2 months.
Tynan quit after 4 1/2 months.
PureDoxyk quit after 6 months.
They all cite impracticality of having to sleep every 4 hours as reason for quiting.
To my awareness there's nobody who upheld the schedule for multiple years.
Even if it's possible to keep up with the Uberman schedule it doesn't seem to worthwhile or those people would probably have continued with it.
As far as short term side effects goes there seems to be mental effects. We unfortunately don't have much more than self reporting about most of the polyphasic sleepers. I will therefore write a bit which is my own understanding from reading about polyphasic sleep.
A lot of polyphasic sleepers report to be more productive during the time they are on the schedule. Polyphasic sleep forces you to do detailed time management or you screw up. The forced time management might account for the gain in productivity.
It's my understanding that the mental state is similar to the state of calmness, that's reached through doing a lot of meditation. The sleepers might therefore perform better on some tasks.
On the other hand long deep problem solving such as making mathematical proofs might suffer.
In a lot of the polyphasic sleep writing you find the claim that those who practice the Uberman schedule are in the REM state when they sleep. It's my understanding that Claudio Stampi showed in Why We Nap: Evolution, Chronobiology, and Functions of Polyphasic and Ultrashort Sleep that this isn't what happens when people adapt to short nap based sleep.
What about sleeping 6 hours? There was a huge study that concluded that people who sleep 6 hours have a longer life span than those that sleep 8 hours.
Correlation isn't causation and this doesn't mean that you will be healthier when you cut your sleep to 6 hours. It however makes it plausible that a 6 hour sleep schedule could be healthy.
If you want to understand more about polyphasic sleep cycles there's a mailing list and a forum.
The forum is run by the Zeo guys who produce equipment to measure sleep states. With a bit of luck they will be able to gather enough data in the coming years to publish a bit on polyphasic sleep schedules.
it also sounds like something that should have evolved naturally if it were possible.
The ability to stay awake for 12 hours without pause is quite valuable. People who practice the Uberman schedule lose the ability.
If you are a hunter gatherer you don't profit very much from being awake at night. It's harder to protect yourself when you don't stay near the fire that your tribe lighted during the night.
I would note that the sleep practices of a person in the western world who sleep in a bed without a nearby fire are very different from the sleep practices according to which we developed.