How to get the content by using get_next_post() and its Id

I am trying te get some cutted Text with substr() Function from the next post data.

This works great but i cant find a way to get just the content from the next post by its ID

<?php $next_post = get_next_post();
  if (!empty( $next_post )){ ?>
      <a href="<?php echo get_permalink( $next_post->ID ); ?>"></a>
      <?php echo get_the_post_thumbnail($next_post->ID, 'medium'); ?>           
<?php } ?>