Studying Physiological and Anatomical Cerebral Effects of Carbon Dioxide and Tilt

Inclusion Criteria: * Male * Ages between 30 to 55 years old * Body Mass Index (BMI) of 20-26 kg/m2 * Weight between 65-85 kg * Height between 158-190 cm * Non-smoker, for at least six months before the start of the study * VO2 max of at least 30ml/kg/min Exclusion Criteria: * History of intracranial pressure elevation * History of abnormal intraocular pressure * Ophthalmological conditions: glaucoma, retinopathy, severe cataracts, eye trauma or implants. * History of diseases of the optic nerve * Pre-existing corneal injury * History of these eye surgeries: implanted lens, corneal transplant, recent (less than 6 months) LASIK surgery * Congenital abnormalities of the anterior chamber * Active eye infections, recent corneal abrasions, inflammation * Intraocular pressure greater than 20 mm Hg * Viral or bacterial eye infection Severe dry eye syndrome * Retinal or choroidal detachment * More than -6,0 dpr (high myopia) * More than +5,0 dpr * Arm or shoulder injury in the past year * Psychiatric conditions including major depression, bipolar disorder or severe anxiety * History of cerebrovascular disease including brain aneurysms, stroke, transient ischemic attack, brain hemorrhage, arteriovenous malformations History of brain tumor, congenital cysts, hydrocephalus, brain injury (requiring hospitalization) * Meningitis/encephalitis * Epilepsy * History of severe hypertension (\> 160/90 mmHg), * Diabetes mellitus * Coronary artery disease * Congestive heart failure * Ventricular or atrial arrhythmias * Autonomic disorders (syncope, autoimmune neuropathy) * Hepatic disease * Renal disease * Chronic infections including HIV, Hepatitis B or C, Lyme disease * History of hematological disease including hemophilia, leukemia, thrombocytosis, thrombocytopenia, myelodysplastic syndrome, autoimmune disease (lupus, rheumatoid arthritis, Sjogren's syndrome) * History of malignancy except for basal cell carcinoma * Chronic back pain (inability to lay in bed for long periods of time) * Conditions that would preclude MRI including severe claustrophobia, pace maker or other metallic implants or devices * Sleep disturbances including: obstructive sleep apnea, narcolepsy, insomnia - Medications that exert cardiovascular, cerebrovascular or psychiatric function (i.e. beta blockers, calcium channel blockers, diuretics, diamox, sedatives) Drug, medication or alcohol abuse (regular consumption of more than 20-30 g alcohol/day)\* * Smoking * VO2 max less than 30 ml/kg/min or greater than 60 ml/kg/min * Vegetarian, vegan * Migraine * Previous psychiatric illness * Hiatus hernia * Gastro-esophageal reflux * Diabetes mellitus * Rheumatic illness * Muscle or joint disorder * Pronounced orthostatic intolerance (\< 10 min standing) * Hyperlipidaemia * Thyroid gland disorder: deviations from normal values for TSH in plasma * Hyper-homocysteinaemia * Hyperuricaemia or hypouricaemia: deviations from normal values for uric acid in plasma. * Hypercalcaemia or hypocalcaemia: deviations from normal values for calcium in plasma. * Iron deficiency * Vitamin D deficiency * Baseline blood gas values deviating from the normal reference values * Elevated risk of venous thromboembolism including deep venous thrombosis, pulmonary embolism * Chronic back complaints * Participation in another clinical study within the last 3 months before start of this study * Criminal record * Stomach sleepers * Any other medical condition that the investigators consider a contraindication to the study procedures that would make it unsafe or confound the measurements.