Yesterday, for the second night, the period just after the waking up was not working: I felt tired and was not comfortable. I had to finally advance the time of the first power nap, after which I felt great again and could stay alert till the time of the second power nap which I didn't need but took anyway (in fact even a bit later than I should have). When came the time of the third core sleep, that I took on time, I felt however very sleepy and was about to take it earlier.
The same situation arose again today. I woke up tired and unwilling. I decided to stay a bit more, extending the sleep by the period of the first power nap (with no assistance of the alarm clock, I just closed my eyes and when I opened them again, 20 minutes had passed). Then I woke up and stood out of bed, but with no enhancement, I felt heavy and inefficient and after drinking a glass of water decided to take the first power nap anyway (that I had already taken technically by staying in bed by as much time a first time).
After that I now feel okay and alert again, although technically I consumed all my sleep credit for today! This points to the fact that the triphasic pattern is not working very well for me. I can sustain reduced amount of sleep but the core one is too short and the short ones are not well distributed during the day.
I don't know yet whether I'll take the power nap(s) or not, my guess is that I'll take one of them in the middle of the time and try to advance the core nap to an earlier time (00:00—04:30).