No-one wants to have to wake up in the night to reapply eye drops, or even be woken up by painful eyes. Therefore, products for use overnight often need to have completely different characteristics to those used during the day.
Two of the most common ingredients are:
Carbomer is a high molecular-weight molecule which makes products much thicker and gel like so they are less likely to drain out of the eyes. The gel texture means they can be used when a longer lasting effect is needed at night or for day-time use. As they are thicker, your vision can initially be slightly blurred but this clears fairly quickly with blinking.
Most carbomer gels come in a tube which needs a preservative although one or two are single use vials. Gels often have a cooling effect interpreted as soothing when they are used.
Paraffin based ointments are usually the preferred option at night especially for severe dry eyes. As you would expect they are very thick which means they last so much longer. They do make the vision blurry for a more prolonged period and can make the skin around your eyes somewhat greasy. Paraffin products are usually preservative-free, it is so thick nothing can grow in it!
Basic carbomer and paraffin based products tend to be very similar. However, you will find that some brands include extra ingredients or features such as: