The hotel is obviously open 24 hours, but the hours of the garden are unknown.
Added to JGarden:
This is a Georgian style 25-story, 439-room highrise on 40 acres of land. The Japanese garden here is striking, but the hotel rooms are also nice. There is a Japanese tatami suite, luxury rooms, business center, conference facilities and all the usual stuff like pools, sauna, data ports, etc. Rates are generally $165-$285. Located in the Buckhead neighborhood at the corner of Peachtree and Piedmont. It was opened as the Hotel in 1990 but was sold in 1997 to Hyatt. The hotel has won several awards and has been rated by Conde Nast Traveler as one of the best 500 Places to Stay in the world. It underwent a US$5.6 million renovation in 2000. Hopefully the Japanese elements have survived.
There are two Japanese gardens, one in the lobby that features two low-volume, 20-foot waterfalls, and a second on the grounds surrounding the hotel. They were designed by Takeo Uesugi and Associates
In the morning mist that lies
Over Akashi Bay,
My longings follow with the ship
That vanishes behind the distant isle.
Akashi no ura no
Fune o shi zo omou.