According to various Vaishnava web sites and various texts, during Ekadashi, one cannot sleep in the night, one cannot sleep during the wee morning hours, and one cannot sleep during the day or else one breaks the fast and there is no merit. So when can we sleep? Are we even allowed to sleep on Ekadashi?
According to Skanda Purana Part six -Book Two - Vaishnav Kanda -Section five - Margashirsha Mahatmya -Chapter 13.Importance of Jagrana during Ekadashi Vrata, One should not sleep during both during day or night in Ekadashi Vrata.
Skanda Purana gives the possible explanation of this as follows.
जागरं ये च कुर्वंति कर्मणा मनसा गिरा ।।
न तेषां पुनरावृत्तिर्मम लोकात्कथंचन ।। 43 ।।
43 Those who observed Jagrana mentally , verbally and physically do not return from my world.
That means the devotees who keep this Vrata and do not sleep during day or night , do not get rebirth on this earth. As already mentioned in that chapter those who perform this vrata get merged with lord as mentioned by he himself.
The purana says because life is transitory ,man get's rebirth on this earth. As day and nights are also transitory .But those who stay awake during both day and night do escape this transition.
प्राप्ताप्येकादशी येषां कलौ जागरणं विना ।।
ते विनष्टा न संदेहो यस्माज्जीवितमध्रुवम् ।। 9 ।।
9 Those who observe ekadashi vrata without Jagrana in kali age without Jagrana are ruined , there is no doubt about it ,because life is Transitory.
Conclusion - Those who is observing the Ekadashi Vrata are not allowed to sleep during entire day or night at all ,because that will ruin the fruits of observing this Vrata of getting salvation , and one will again be reborn , if he failed to do so.