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One "classic" presentation of metastatic bone cancer is back pain that "tends to be worse at night and may get better with movement" - American Cancer Society.
Googling "back pain worse at night from metastatic bone cancer" leads me to a case study, but I can't find a proposed mechanism for why the back pain is specifically worse at night.
Not sure why I didn't think of this earlier, but back pain that is worse at night due to metastatic cancer to the spine is caused by tumor compressing spinal cord when lying on your back. Anything that worsens the compression, such as "when you cough, sneeze or go to the toilet", will also worsen the pain.
Source: https://www.cancerresearchuk.org/about-cancer/coping/physically/spinal-cord-compression/about
