Catalyzes the final one or two reductions in tetra-hydrobiopterin biosynthesis to form 5,6,7,8-tetrahydrobiopterin. The enzyme also catalyzes the reduction of benzil to (S)-benzoin. L-erythro-7,8-dihydrobiopterin + NADP(+) = H(+) + NADPH + sepiapterin (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin + 2 NADP(+) = 6-pyruvoyl-5,6,7,8-tetrahydropterin + 2 H(+) + 2 NADPH (S)-benzoin + NADP(+) = benzil + H(+) + NADPH Homodimer. Belongs to the sepiapterin reductase family.