Can't get the $current_user object in functions.php included file

I want to get the currently logged-in user in the “wp_login’ hook.

function mineral_child_login_updates() {
    // WP database handler
    global $wpdb, $current_user;
    $current_user = wp_get_current_user();
    $table_name = $wpdb->prefix . "cz_registered_users";
    $wpdb->update( $table_name,
	array(
		'user_last_login' => date("Y-m-d H:i:s"),
	),
	"WHERE user_login= " . $current_user->user_login
    );
}
add_action('wp_login', 'mineral_child_login_updates');

Using a child theme I put the above function inside functions.php and it works. If I create a separate file for it in another folder that I insert in the functions.php file it doesn’t work, the $current_user properties returning null. What am I doing wrong?
PS: no problem with the database …