Worsening Concentration Impairment

I am a PhD student in the UK working on philosophical & mathematical logic, which requires prolonged periods of intense analytic thinking. I have found that over the last year or two, my ability to concentrate has degraded considerably. I' constantly reading the last sentence of paragraphs or proofs with no idea what was happening in the beginning of the segment. I find the same is happening with my memory. It used to be the case that I could tell somebody precisely what had happened in a class 6 months previously, including personal anecdotes, but now I feel I have a hard time condensing the data from a seminar last week to make it coherent in summary.(I mention this because I can remember some details, but struggle to make them combine as part of a coherent narrative.)
I'm really worried about this, because doing sums and analyzing their implications is literally my livelihood, and obviously the prospect that this ability is getting gradually worse is a bit scary.
I smoke and drink, but neither excessively (about 6 cigs a day) and I don't frequently get 'drunk',I more prefer a glass of wine with a meal or a pint during a match. My sleeping isn't too bad-I don't sleep as many hours as most per night, but that's always been the case, and is the same with most of my family. My grades have always been fairly good, (but I worry as to how I do now...). I definitely don't think I'm depressed, I have a great group of people around me in my dept and my family at home are wonderfully supportive. Any help would be greatly appreciated- I'd be willing to try any suggestions at this rate.
Thanks
Clare

[Rich99]

""""My sleeping isn't too bad"""""  which is like saying,  my sleep isn't the best.    Lack of enough sleep can make for all kinds of problems including coping with stress,  memory and the ability to concentrate clearly.      It's really common for students with heavy heavy schedules to not get enough rest.