Extended smart phone use creates problems because working distances are so close. Sustained focus (see above) and extended convergence (in-turning of eyes) cause fatigue and strain. Proper eye wear and taking breaks are the best solutions.
Progressive lens users also have postural issues. At a desk, the ideal spot to view a desktop screen is partway down the lens… this causes users to lift their chin for extended periods resulting in aches and pains. There are products that allow for more comfortable postures (e.g. Essilor Digitime or Zeiss Officelens). These lanes very helpful to reduce strain and address multiple working distance issues. If a separate set of glasses is not practical, setting the computer monitor as low as possible is important (i.e. with straight ahead gaze, the top of your computer screen should be at eye level)