"One who desires to live long should not climb the back of a cow or a bull, should not allow the smoke from a funeral pyre to touch his body, should not sit on the bank of a river, other than the Ganga, at dusk, should not allow the rays of the rising sun to touch his body and should not sleep during the day"

"One who desires to live long should not climb the back of a cow or a bull, should not allow the smoke from a funeral pyre to touch his body, should not sit on the bank of a river, other than the Ganga, at dusk, should not allow the rays of the rising sun to touch his body and should not sleep during the day" ~ Skanda Purana
Since at sunrise the percentage of Raja-Tama-predominant particles in the atmosphere is more than in the sun rays, avoid contact with sun rays. The rays of the rising sun begin the process of disintegration of the Raja-tama-predominant particles, which have accumulated in the environment at night. This friction generates lot of heat in the environment. In addition, since the proportion of Raja-Tama-predominant particles in the environment during sunrise is more than in sun rays, these particles gain some amount of momentum along with the sunrays. Hence, we should avoid contact with these sunrays.
However during worship of the rising sun, it is said that rays of the rising sun should touch the body; some even recommend viewing them. To achieve control over the Tej-tattva, Shakti worshippers undertake spiritual practice by performing Traṭak (Staring at the sun) or penance by causing pain to the body by exposing it to the sun rays. However, for those following Sahajyoga (State of self-realisation produced by kundalini awakening (Dormant spiritual energy in the subtle-body), this is not necessary. ‪Worship at sunrise means undertaking maximum chanting during that period without coming into direct contact of the sun rays.' Intensity of the Raja-Tama-predominant waves being high during the transition period at sunrise and sunset, it is essential to undertake spiritual practice at that time. 'Raja-Tama-predominant waves start decreasing in the transition period during sunrise and start increasing in the transition period during sunset. These two being the main periods of concentration of Raja-Tama-predominant waves, to avoid adverse effects the individual is advised to undertake spiritual practice at sunrise and sunset to enhance its sattvikta. Once the sattvik waves in the individual increase, they create a protective sheath around the individual, reducing the ill-effects of sunrise and sunset.' 
According to Puranas, Sleeping at sunrise makes us susceptible to acquiring of Patal (Hell region). 'After sunrise, sattvik waves reach the earth. During this period, Deities in the form of fragrance descend onto the earth. At this time if an individual sleeps instead of performing spiritual practice, then the physical body gets charged with distressing vibrations from Patal. These Raja-Tama-predominant waves are then emitted to the atmosphere, blocking the entry of sattvik waves onto the earth. Hence, such an individual may have to face the wrath of inferior Deities. That is why, it is said that the individual becomes susceptible to committing a sin that is, acquiring of Patal.' Continuing sleeping at sunrise results in a sin because of not availing the benefit of the sattvik waves. 'Sleeping during sunrise deprives us from chanting and benefitting from the sattvik waves, thus making us a slave to sleep. It is therefore considered to be a sin.'
Har Har Mahadev ~ Om Namah Shivaya!