Based on peer reviewed, double-blind, placebo-controlled clinical trials and studies:

L-theanine and caffeine in combination affect human cognition as evidenced by oscillatory alpha-band activity and attention task performance.

S.P. Kelly, M. Gomez-Ramirez, J.L. Montesi, J.J. Foxe.

The combined effects of L-theanine and caffeine on cognitive performance and mood.

G.N. Owen, H. Parnell, E. A. De Bruin, J. A. Rycroft.

The combination of L-theanine and caffeine improves cognitive performance and increases subjective alertness.

T. Giesbrecht, J.A. Rycroft, M.J. Rowson, E.A. De Bruin

The effects of L-theanine, caffeine and their combination on cognition and mood.

C.F. Haskell, D.O. Kennedy, A.L. Milne AL, K.A. Wesnes, A.B. Scholey.

L-theanine and caffeine improve task switching but not intersensory attention or subjective alertness.

S.J. Einöther, V.E. Martens, J.A. Rycroft, E.A. De Bruin