What are the adverse effects from concentrating for prolonged periods of time without break (4-8 hrs)? (study, reading, solving puzzles)

Some things.... It is important to take some breaks to walk around, stretch and get nourishment. Otherwise you may get muscle tightness in your neck and upper back and even your legs. This could lead to headache and in very rare circumstances blood clots in the legs (although highly doubtful within 8 hrs.