We started from Yosemite Valley trail head parking lot at 1AM. Considering how hot it was when we came down (92F), I would recommend starting at that time. We started from the car with our head lamps on and they stayed on until we were nearing the sub-dome. The hike up until the sub dome is very straight forward, but the path is steep and very rocky at points. I would recommend taking the John Muir trail going up if you are starting at night (unless the river is low), otherwise you will be hiking in went clothing at night if you took the Mist trail.
Once you reach the sub dome you will be greeted with some steep and slightly exposed chiseled steps and gain a good bit of elevation here. Below is a picture from the sub dome looking back at the trail.
Congratulation, you've made it to the Half Dome cables. I hope you remembered your gloves and good shoes. The rock is slippery regardless of weather condition, so your upper body and grip strength will get you through this. The climbing isn't physically hard, but this is a no fall zone. Move your hands, take some steps, and rest on the wood as needed. Remember to stay to the right and let others pass.
You've made it to the top of half dome! Enjoy the views, takes some pictures, hydrate and eat. You are officially half way done with your hike.
Its time to start your way down the cables, some people prefer to walk backwards. But I found the down climb easier than going up. I faced forward and used both hands to control my decent. Left on left and right on right. Just move with purpose and you'll be fine. Heading down is isn't as bad as you thought (even if it looks scary).
Now that you've made it down, you still have nearly 9 miles until you get back to the car. On your way back, as the temperature increases, the crowds are everywhere and your fellow hikers are struggling in the heat. You will be very glad you opted to start at 1AM.
On your way down, even though the Mist trail will be crowded the refreshing cold water will be well worth it.
Our time could have been a bit faster if I didn't spent 45 minutes taking some night photography photos.