A method includes: (a) obtaining information about a period of a fluctuation cycle in an autonomic nerve of a user at rest; (b) inducing the user to breaths in synchronization with the period of the fluctuation cycle in the autonomic nerve according to the obtained information; and (c) synchronizing a fluctuation in a diameter of a pupil of the user with the period of the fluctuation cycle in the autonomic nerve at the same time of (b) according to the obtained information. The period of the fluctuation cycle in the autonomic nerve is a period of a fluctuation cycle in the diameter of the pupil, a period of a fluctuation cycle in a heart beats of the user, or a period calculated from the period of the fluctuation cycle in the diameter of the pupil and the period of the fluctuation cycle in the heart beats.