Catalyzes the final one or two reductions in tetra-hydrobiopterin biosynthesis to form 5,6,7,8-tetrahydrobiopterin. 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 kcat is 9.7 sec(-1) with sepiapterin as substrate (PubMed:10350607). kcat is 11.1 sec(-1) with NADPH as substrate (PubMed:10350607). Homodimer. In vitro phosphorylation of Ser-46, Ser-196 and Ser-214 by CaMK2 does not change kinetic parameters. Belongs to the sepiapterin reductase family.