We did both day and night visits on our trip in April, and each is different and wonderful.
What I would recommend is seeing the entire site in the morning (including the Nasrid palaces) with a general ticket, either with the audioguide or with a tour guide so that you can learn the full history and impact of what you're seeing. It's so impressive. And yes, you can easily spend 4-5 hours during this visit. Don't rush.
Consider a tapas tour for lunch. Granada is famous for its tapas; they are still served free with your drinks at most of the bars in the city. We used granadatapastours.com and enjoyed our tour very much. Or just walk around the historical area and find a menu you like.
In the afternoon, go see other sites in Granada. Yes, Rick's old town walk covers the central ones. The cathedral is beautiful (light and airy, a totally different feel from Spain's other cathedrals), and right next door is the Capilla Real with the tombs of Isabel and Fernando. Afterward, you can wander through the Alcaiceria for souvenir shopping. It's largely touristy now, but still fun, and it's interesting to see the architecture when you understand its history (see Rick's book, or google for more; even though the buildings burned and have been rebuilt, it was fascinating for me to learn about the trade aspect of Granada's history).
When you are done sightseeing, grab some tapas or dinner. Los Diamantes, along the Plaza Nueva, is awesome for seafood and is right next to the walking path up to the Alhambra (or you can catch at taxi at the stand out front).
Then head up for a night visit to the palaces (starts 10 p.m). It's magical at night. Plan on spending about an hour or an hour and a half just to wander around. Really beautiful.
You need to purchase your Alhambra tickets ASAP -- they are showing limited availability for your travel dates.
P.S. -- on the 19th, take the time to wander around the the Albaicin. It's a really special area with a unique vibe. Make sure to get ice cream at Los Helados de San Nicolas. If you want a really memorable dinner, I recommend Carmen Aben Humeya, just off the Mirador San Nicolas. It has a stunning view of the Alhambra, which is beautiful at night, and the food is to die for. It's expensive, but worth the whole experience. You might need a reservation for dinner.
Have a great trip!