A couple of quick google searches would tell you that the important part of sleep is the REM (Rapid Eye Movement) sleep.
REM stands for rapid eye movement. During REM sleep, your eyes move quickly in different directions. That doesn't happen during non-REM sleep.
Usually, REM sleep happens 90 minutes after you fall asleep. The first period of REM typically lasts 10 minutes. Each of your later REM stages gets longer, and the final one may last up to an hour. Your heart rate and breathing quickens.You can have intense dreams during REM sleep, since your brain is more active.(1)
During an eight-hour sleep pattern, the average adult only spends approximately 1.5 hours (or 20% of total sleep time) in rapid eye movement (REM) sleep. REM sleep is important because it is the restorative part of our sleep cycle. Typically, you begin the sleep cycle with a period of non-REM sleep followed by a very short period of REM sleep. The period of non-REM sleep is made up of stages 1 to 4. Each stage can last from 5 to 15 minutes. A completed cycle of sleep consists of a progression from stages 1 to 4 before REM sleep is attained, then the cycle starts over again.
However, if your REM sleep is disrupted even one night, your body won’t follow its normal circadian sleep cycle (“inner clock”) progression. Instead, you will slip directly into REM sleep as a result of not getting the right amount of sleep the night before. You will also go through extended periods of REM sleep until you “catch up” on this stage of sleep. Poor sleep cycles can cause grogginess, a lack of concentration and more.(2)
During day, brain might not get enough time to restore itself as one can hardly sleep for 2-3 hours during day. Though your body might get refreshed after this, your brain might not. Also, as @TheGreenOne says, there are lot more chances of day sleep getting disturbed than night sleep. Another thing is that the brain is habitual to sleep at its normal time (night) (see SCN), so it won't be very easy to get in/out of the habit to sleep during day.