I am a pretty good sleeper - generally crash fast and hard at night. I have also taken to using earplugs to block out noises (especially the sound of traffic - I live on a moderately busy street). I was busy last night and didn't hit the sack until 11:30 PM or so. I was heading out to O'Hare to catch a 6:10 AM flight to Atlanta in the morning, so I set the clock for 3:30 AM. I spent 30 minutes trying to forget about the short ration of sleep I was allocating to myself and finally drifted off. It seems like my alarm screamed 10 seconds later. I yanked out my earplugs and made it to O'Hare, caught my flight and sat in fairly intense business meetings all day. I have the classic symptoms of sleep deprivation - headache, cotton mouth, concentration difficulties, etc. It is a lot like being hungover without the fun misbehavior of excessive alcohol consumption during the preceding evening.
I have a pile of documents to read, but it looks too tough to tackle right now. I have to rack out now and try again tomorrow. I can function reasonably well on six hours of sleep, but I fall apart on less than 4 hours.