private function tokenRequest($namespace, $parameters)
    {
        $client = new Client([
            'headers' => [
                'content-type' => 'application/x-www-form-urlencoded',
                'Authorization' => 'Basic '. base64_encode($this->clientId . ':' . $this->secret),
            ]
        ]);

        return $client->post(self::FITBIT_API_URL . $namespace . '?' . http_build_query($parameters));
    }