Eli was comparing the temperatures at a desert site and a coastal site that have the same latitude. he noticed that the nighttime temperature dropped significantly at the desert site but only slightly at the coastal site. this difference is mostly caused by

High heat capacity of the ocean (water body) in coastal areas than that of sand (land) in deserts.

Explanation:

The coastal areas have water body (ocean) and the desert are devoid of water bodies. This basic difference in the geography of the two native places makes the entire difference  in their   temperature at day and night time.

The heat capacity of water is much higher than that of the sand in the desert and due to this reason the time taken by the ocean/sea to heat and to cool is much higher than that of desert.  

Therefore, though the day time temperature in desert is higher than that of coastal area , the night time temperature gets lower than the ocean.