The powers of the sympathetic nerve are neither entirely physical nor spiritual, but are between the two (systems). The nerve is connected with both. Its phenomena shall be perfect when its spiritual and physical relations are normal.
(‘Abdu’l-Bahá, Tablets of ‘Abdu’l-Bahá v2, p. 309)
You should certainly safeguard your nerves, and force yourself to take time, and not only for prayer meditation, but for real rest and relaxation.
(Shoghi Effendi, Lights of Guidance, p. 296)