Thoracic Outlet Syndrome (TOS): This condition involves compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib). Symptoms can worsen when laying flat due to increased pressure on these structures, leading to tingling in the hands and feet.

Cervical Spondylosis: This condition involves wear and tear on the discs in the neck, which can lead to compression of the spinal cord or nerves. Laying flat can exacerbate symptoms by altering the cervical spine's alignment, causing tingling sensations in the extremities.
Lumbar Spondylosis: Similar to cervical spondylosis, but affecting the lower back. Compression of the nerves in the lumbar region can cause tingling in the feet and, less commonly, the hands, especially when the body is in a flat position.
Vitamin B12 Deficiency: A deficiency in vitamin B12 can cause neurological symptoms, including tingling in the hands and feet. While the position of the body might not directly influence the deficiency, symptoms could be more noticeable when laying flat due to increased blood flow or relaxation of the body.

Multiple Sclerosis (MS): An autoimmune disease that affects the brain and spinal cord, leading to a wide range of neurological symptoms, including tingling. MS can present with a variety of symptoms that might seem unrelated, making it crucial not to miss, despite its lower likelihood.
Spinal Cord Compression: This is a serious condition that requires immediate medical attention. Compression of the spinal cord can cause a range of symptoms, including tingling in the hands and feet, which might worsen in certain positions, such as laying flat.

Peripheral Artery Disease (PAD): While more commonly associated with leg pain during walking, in rare cases, PAD could potentially cause tingling in the feet when laying flat due to decreased blood flow.
Guillain-Barré Syndrome: A rare autoimmune disorder in which the body's immune system mistakenly attacks part of its peripheral nervous system, leading to muscle weakness, and sometimes tingling sensations in the hands and feet. The symptoms could potentially be position-dependent in some cases.
Neurodegenerative Diseases (e.g., Amyotrophic Lateral Sclerosis - ALS): These diseases affect the nerve cells responsible for controlling voluntary muscle movement. While rare, they can cause a variety of neurological symptoms, including tingling, which might be noticed in specific positions.