55-year-old woman has difficulties in achieving adequate control of her T2DM, hypertension and lipidemia. She is confirming her compliance with medications. There is no change in dietary habits or Diarrhea. She is reporting excessive daytime tiredness and sleepiness. Her husband recalls frequent loud snoring. Her BP 160/95mmHg, HR 100/min, O2 saturation 93% on room air, RR 20/min. Hight 155cm Weight 110 kg. Pulmonary examination showed crowded upper airway, decreased breath sounds at bases. Which investigation will reveal the reason behind lack of response to treatment of her illnesses?