12-15:
The Barrens (Circuits 1-2)
15-16:
Stonetalon Mountains (Circuit 1)
16-20:
The Barrens (Circuits 3-4)
20-21:
STonetalon Mountains (Circuit 2)
21-22:
Ashenvale (Circuit 1)
22-23:
The Barrens (Circuit 5)
23-25:
Stonetalon Mountain (Circuit 3-4)
25:
The Barrens (Circuit 6)
25-26:
Thousand Needles (Circuit 1)
26-27:
Ashenvale (Circuits 2-3)
27-29:
Thouand Needles (Circuits 2-3)
29-30?:
Hillsbrad Foothills (Circuit 1)
30:
Arathi Highalands (Circuit 1)
30-31:
Stranglethorn Vale (Circuit 1)
31-32:
Thousand Needles (Circuit 4)
32-34:
Desolace (Circuits 1-3)
34:
Dustwallow Marsh (Circuit 1)(Not even really a circuit)
34-36:
Stranglethorn Vale (Circuits 2-3)
36-37:
Arathi Highlands (Circuits 2-3)
37:
Alterac Mountains (Circuit 1)
37:
Thousand Needles (Circuit 5)
37-38:
Dustwallow Marsh (Circuits 2-3)
38-40:
Stranglethorn Vale (Circuit 4)
40-41:
Badlands (Circuit 1)
41-42:
Swamp of Sorrows (Circuits 1-2)
42-43:
Stranglethorn Vale(Circuit 5) DOES IT EVER END?!
43:
Dustwallow Marsh (Circuit 4)
43-44:
Tanaris (Circuit 1)
44-46:
Feralas (Circuits 1-3)
46:
Azshara (Circuit 1)
46-47:
The Hinterlands (Circuit 1)
47:
Stranglethorn Vale (Circuit 6)
47-48:
Searing Gorge (Circuits 1-2)
48:
Swamp of Sorrows (Circuit 3)
48:
Dustwallow Marsh (Circuit 5)
48-49:
Feralas (Circuit 4)
49-50:
Tanaris (Circuit 2)
50:
Azshara. (Circuit 2)
50:
The Hinterlands (Circuit 2)
50-51:
The Blasted Lands (Doesn't follow the Circuit system)
51-52:
Un Goro Crater (Circuits 1-2)
52-53:
Burning Steppes(Circuit 1)
53:
Azshara (Circuit 3)
53:
Felwood (Circuit 1)
53-54:
Winterspring (Circuits 1-3)
54-55:
Felwood (Circuits 2-3)
55:
Silithus (Circuits 1-2)
55-56:
Western Plaguelands (Circuit 1)
56-57:
Eastern Plaguelands (Circuits 1-2)
57-58:
Western Plaguelands (circuits 2-3)
Outland (level ranges are best guesses):
58-63:
Hellfire Peninsula
60-64:
Zangarmarsh
62-65:
Terokkar
64-67:
Nagrand
65-68:
Blade's Edge Mountains
67-70:
Netherstorm