Sleeping is a form of rest which happens due to tiredness or tummy full food. so have you ever checked those day's work routine and dinner, if no, monitor that, you will surely get an idea about this deep sleeps.
Are you eating a large amount of carbohydrate foods in these periods or your intake of food is the same during and after the 'sleep spells'? High amounts of carbohydrate foods at one meal tend to get people tried due to the increase/drop of the sugar levels in the blood. Maybe look at periods of stress in your job due to certain timetables, such as an accountant very busy at the beginning of a year due to income tax time, etc. Our bodies get fatigued from high amounts of stress. If all above are no, then suggest seeing your Doctor to be checked. Thank you for your question.