The best part of waking up.... is Russian Roulette in your cup!
That was ah goo d'wahn!
The reason from what I've read may be linked to hormonal conditions. For some reason in the morning the blood stream is high in a particular enzyme which has to do with adrenaline binding to receptors. Later in the day, there are other enzymes present in higher levels which mitigate the effects of the adrenaline-binding hormones. Adrenaline in turn has a tendancy to sometimes be correlated to production of "stress hormones" like cortisol, which is in turn linked statistically to heart ailments.
It could be that the same hormonal factors related to increased adrenaline binding also affect the amount of binding to receptors of caffeine molecules.
http://www.webmd.com/heart-disease/news/20110427/heart-attacks-in-the-morning-are-more-severe#2 It would not be surprising if the poor fellow who is the subject of the article died in the morning and may have had a hidden heart condition of which he was unaware. There is evidence that this is a common reason people die unexpectedly from heart attacks (when they have previously shown no other inclination toward problems).
NOTE: There is also a statistic (can't remember where I read it) which said that people who have just had sex are for at least an hour or two afterward in the lowest risk category for heart attacks (unless they have a prexisting condition and are not taking proper medicine to treat it). This apparently has to do with endorphines which are released in the love-making process, which tends to relax the heart and dialate blood vessels, reducing two important factors in myocardial infarction.
NOTE#2 : I had a yoga instructor who believed that low oxygen level in the bloodstream is also a high-risk factor for heart attacks, although I am not aware of any studies which cite this. It makes sense though from a practical perspective, since low oxygen would tend to inhibit efficiency of metabolism including the endocrine system and the circulatory system. People tend to also breathe shallowly and unevenly when upset or angry, which is not conducive to good health. My instructor often said that "People don't breathe properly. All of that free oxygen going to waste.".
As far as increased danger immediately after waking up - I will look for the source. I read that there is statistical evidence that the body is in a hyper-sensitive state for at least a half hour after awakening. This has to do with blood pressure being low from normal sleep and needing at least a half-hour of waking to come up to normal level. Hormonal activity is linked to blood pressure for some reason. Very low or very high blood pressure affects the endocrine system significantly.
