I finished the second week of biphasic sleep.
During the first week, it was impossible for me to be productive after the 90 minute nap, but now I feel energetic enough to get stuff done during that period. My eyes still feel a little heavy though, but not enough to prevent book reading.
I think my body is still adapting to the 90 minute nap. I have yet to wake up before the alarm, but it's getting easier and easier to get out of bed. I've been waking up from core sleep before the alarm, and it appears that my natural cycle is somewhat under 90 minutes (but not by much).
Biphasic sleep has given me a small boost in overall energy. I wake up from my core sleep feeling nicely refreshed. But my energy level is in a more fragile state. Delay the nap by several hours? Bad idea. Eat a big dinner? Terrible idea. So if I want to enjoy the energy boost of biphasic sleep, it looks like I have to be extra careful of things like carb crashes, and I need to stick pretty hard to the sleep schedule.
For the past few months I haven't had many vivid dreams (this is something I dearly miss, by the way. I used to have beautifully vivid dreams on a nightly basis a few years back. I'm currently trying to find a way to bring those back.) But when I started biphasic sleep, I also started a dream journal and have been practicing the most basic dream recall techniques. So far, nothing. Dream recall has decreased a small amount since I started biphasic sleep.
Other than that, so far so good. The extra 2 hours in the day are worth the effort of being biphasic.