referring to the several views of the drawings , several embodiments of the bed sheet set of the present invention are shown , wherein the bed sheet set is generally indicated as 10 . in each of the embodiments , the bed sheet set 10 includes a lower sheet 20 and an upper sheet 30 which , in conjunction , are structured to enclose a mattress 100 having a top sleeping surface 102 , a bottom side 104 and vertical sides 106 . a first preferred embodiment of the bed sheet set 10 is shown in fig1 - 3 a , wherein the mattress 100 is partly encompassed by the lower sheet 20 . the lower sheet 20 is preferably structured to include a rectangular main bottom panel 21 and upstanding sidewalls 24 about the periphery . the bottom panel 21 has an inner surface 22 and an outer surface 23 . the juncture between the bottom panel 21 and side walls 24 may be of conventional construction with seamed edges to facilitate a snug , uniform fit about the bottom 102 and sides 106 of the mattress 100 . the lower sheet 20 may be of a woven or knitted cotton or other conventional threads or yarns with a thread count determining the softness and air permeability of the panel . a fastener 25 extends about the peripheral zone 26 on the sidewalls 24 of the lower sheet 20 . the location of the fastener 25 is such that when the lower sheet 20 is fitted onto the mattress 100 , the fastener 25 is disposed about the sides 106 of the mattress . the fastener 25 may be of a series of cooperating fasteners , such as opposite sides of a zipper , cooperating button and buttonholes , male and female snaps , or hook and loop fabric such as velcro or releasable cohesive materials . the fasteners should be of materials that will withstand numerous use and laundering cycles . in one preferred embodiment , the cooperating fasteners are defined by non - corrosive zippers which join the lower sheet 20 and upper sheet 30 about their entire correspondingly positioned peripheral edge zones to completely enclose the mattress 100 therein . the upper sheet 30 is defined by an air - permeable main top panel 31 having a top outer surface 32 , an inner surface 34 , and a peripheral extremity 35 to completely overly the top sleeping surface 102 of the mattress 100 . in a preferred embodiment , the peripheral extremity 35 of the upper sheet 30 extends partially down the sidewalls 106 of the mattress , just below the top peripheral edge 108 with the inner surface 34 facing the top sleeping surface 102 of the mattress 100 . alternatively , the upper sheet 30 may be sized to cover the top surface 102 of the mattress only , with the peripheral extremity of the upper sheet aligned with the top edge 108 or positioned just inside the top edge 108 of the mattress 100 . there is a first fastener 37 attached to the peripheral edge zone 36 of the upper sheet 30 . this fastener is the cooperating opposite part of the fastener 25 described above . a fly or flap 38 extends from the peripheral edge zone 36 to cover the cooperating fasteners 25 , 37 of the lower and upper panels when the fasteners are attached . the upper sheet 30 may be of the same or similar material as the lower sheet 20 with similar thread count and air permeability . the threads or yarns may be natural or synthetic and , possibly , hydrophobic to increase the resistance of the panel to penetration of fluids . the lower and upper sheets may be deployed , alone , during waking hours , when a person using the bed may be capable of managing the bodily functions . during this period , the air permeability of the bed sheet set 10 is normal without excessive heat buildup . the exposure of the upper sheet 30 will dictate changing on a regular basis , for example every three days or once a week . at such time , the cooperating fasteners 25 , 37 will be unfastened , thereby allowing the upper sheet 30 to be removed and laundered while the lower sheet 20 remains in place and undisturbed . as seen in fig1 - 3 b , the bed sheet set 10 further includes a moisture proof major panel 40 adapted for positioning between the top sleeping surface 102 of the mattress 100 and the inner surface 34 of the top panel 31 . the moisture proof major panel 40 is sized , structured and configured to cover at least a portion of the area of the top sleeping surface 102 and , in a preferred embodiment , the moisture proof major panel 40 covers the entire mattress top sleeping surface 102 . the major panel 40 is structured to include an absorbent top side 42 and a waterproof underside 44 . the absorbent top side 42 faces the inner surface 34 of the top panel 31 while the waterproof side 44 faces the top surface 102 of the mattress . in one embodiment , the moisture proof major panel 40 is selectively positionable between the upper sheet and the mattress 100 and may be removed when desired to allow greater ventilation and to avoid excessive heat buildup . during sleeping hours , or when it is otherwise desired to protect the mattress , the major panel 40 can be inserted between the upper sheet 30 and mattress 100 . as best seen in fig1 , cooperating releasable fasteners 46 and 48 are provided on the absorbent side 42 of the major panel 40 and the inner surface of the top panel 31 , respectively , to facilitate attachment of the major panel 40 to the upper sheet 30 in a manner which prevents unwanted traveling or bunching up of the major panel 40 below the upper sheet 30 due to movement of the person lying on the bed or crib mattress . in another embodiment , as seen in fig3 b , the moisture proof major panel 40 is permanently sewn to the top panel 31 , on the underside 34 . in this instance , the major panel 40 may be stitched or otherwise attached to the inner surface 34 of the top panel 31 or , alternatively , sandwiched between two sheet layers making up the top panel 31 and stitched or otherwise secured in place . in either embodiment , as described above , when it is necessary to change the upper sheet 30 , both the upper sheet 30 and major panel 40 are removed from the mattress and lower sheet 20 by unfastening the cooperating fasteners 25 , 37 . the major panel 40 may then be laundered along with the upper sheet 30 or , in the embodiment of fig1 - 3 a , if the major panel 40 has not been soiled it can be simply replaced on the top 102 of the mattress 100 for subsequent attachment to the inner surface 34 of the top panel 31 once the upper sheet 30 has been washed and dried . in a further embodiment , as shown in fig4 and 5 , the lower sheet 20 is provided with releasable fasteners 50 on the bottom outer surface 23 . a minor panel 60 , as shown in fig4 and 5 , for covering a portion of the upper sheet 30 is shown with corresponding fastener elements 62 for releasably connecting to the fastener elements 50 on the bottom of the lower sheet 20 . the minor panel 60 is waterproof with an absorbent layer sandwiched between opposing fabric panels which may be formed of cotton , synthetic or other conventional bed linen materials . alternatively , the minor panel 60 may be made of a combination of an absorbent fabric on the top with a liquid impervious film backing on the bottom side . the minor panel 60 is intended to protect a remainder of the bed sheet set 10 , including the upper sheet 30 , major panel 40 ( if used ) and lower sheet 20 , as well as the mattress 100 , from liquid discharges , such as drooling , nasal discharges or beverage spills ( e . g . leaking nursing bottles ). the minor panel 60 may be of varying sizes to cover different portions of the top panel 31 of the upper sheet 30 in accordance with the particular needs of the user . the minor panel 60 helps to reduce the number of required bed linen changes and laundering , by protecting the remainder of the bed sheet set 10 . when the minor panel 60 becomes soiled , it can be simply removed , by unfastening the cooperating fasteners 50 , 62 on the underside of the lower sheet 20 , below the mattress , and removing the minor panel 60 from the bed sheet set 10 and mattress 100 . the minor panel 60 can then be conveniently laundered and replaced as needed . further , the use of the minor panel 60 will allow the caregiver or the user to remove only the soiled minor panel portion of the bed sheet set 10 and replace it with another clean minor panel quickly and easily without disrupting the remainder of the bed sheet set 10 and mattress . referring to fig6 and 7 , yet a further embodiment of the bed sheet set 10 is shown wherein the lower sheet 20 ′ and upper sheet 30 ′ are permanently joined together in the form of a casing 70 a which is sized , structured and configured to enclose the mattress 100 . a flap portion 72 of the casing 70 a unfolds to reveal an opening 74 which allows the end of the mattress to be slid within the interior of the bed sheet casing 70 a . the casing 70 a can then be pulled over and around the sides of the mattress 100 so that the mattress is completely tucked within the casing 70 a defined by the combined lower and upper sheets 20 ′, 30 ′, respectively . the flap 72 can then be closed and secured with cooperating fasteners 76 , 78 , such as hook and loop fasteners , to completely encapsulate the mattress within the bed sheet enclosure . in this particular embodiment , the moisture proof major panel 40 may be used , as desired , and may be provided as an independent component , as shown and described in connection with fig1 - 3 a , or , alternatively , the major panel 40 may be permanently stitched or otherwise attached to the inner side of the upper sheet 30 ′. fig8 shows a further embodiment of the bed sheet enclosure 70 b defined by the combined lower sheet 20 ′ and upper sheet 30 ′ which are permanently secured together as an integral one - piece component , similar to the embodiment of fig6 - 7 . in the embodiment of fig8 , the flap 72 is located at the end of the bed sheet enclosure , partially extending down the end sidewall 24 ′, thereby providing a convenient opening 74 when the flap 72 is unfolded . this allows the bed sheet enclosure 70 b to be slipped over the end of the mattress 100 and pulled along the length of the mattress so that the mattress is tucked and secured in enclosed relation within the bed sheet enclosure . the flap 72 can then be secured closed , using the cooperating releasable fasteners 76 , 78 , in the same general manner as described in connection with the embodiment of fig6 - 7 . again , the major panel 40 may be provided as a separate component and inserted between the upper sheet 30 ′ and mattress 100 , or , alternatively , the major panel 40 may be permanently stitched or otherwise secured under the inner side of the upper sheet 30 ′ in the manner as described in the previous embodiments of fig1 - 3 b . the instant invention provides a method of safely securing a bed sheet set to a mattress while allowing for quick changing of the bed sheets for laundering as necessary comprising the steps of : ( a ) providing the set of bed sheets with an absorbent porous lower sheet having an attached continuous fastener ; ( b ) securing the lower sheet about a mattress such that the fastener is disposed about the perimeter of the mattress ; ( c ) providing the bed sheet set with an absorbent porous upper sheet having a first attached continuous fastener ; ( d ) providing a moisture proof panel sized , structured and configured to cover at least a portion of the area of the top of the mattress ; ( e ) releasably securing the continuous fastener on the lower sheet to the continuous fastener on the upper sheet to enclose the mattress therein ; ( f ) selectively placing the moisture proof major panel between the upper sheet and the top of the mattress ; and ( g ) removing the upper sheet and the major panel from the mattress and the lower sheet by unfastening the cooperating continuous fasteners on the upper sheet and the lower sheet , thereby allowing the upper sheet and / or the major panel to be laundered and replaced while the lower sheet and mattress remain stationery , in place and undisturbed . further , the instant invention provides a method of changing the air permeability of a set of bed sheets during use comprising the steps recited above , wherein the air permeability of the bed sheet set can be changed by selectively removing or inserting the moisture proof major panel between the upper sheet 30 and the mattress 100 . the invention further provides a method of extending the period of time between changes of a set of bed sheets comprising the steps of : ( a ) providing the set with an absorbent porous lower sheet having an attached continuous fastener and a third attached fastener ; ( b ) securing the lower sheet about a mattress such that the fastener is disposed about the perimeter of the mattress ; ( c ) providing the set with an absorbent porous upper sheet having a first attached continuous fastener and a second attached fastener ; ( d ) releasably securing the continuous fastener on the lower sheet to the first continuous fastener on the upper sheet to enclose the mattress in encapsulated relation between the lower sheet and upper sheet ; ( e ) providing the bed sheet set with a waterproof major panel and a plurality of waterproof minor panels each having discreet edges and a fastener attached about their edges ; ( f ) selectively placing the major panel between the upper sheet and the top of the mattress and releasably securing a fastener on the waterproof major panel with the second fastener on the upper sheet ; ( g ) providing the bed sheet set with a plurality of minor waterproof panels each having a releasable fastener ; ( h ) releasably securing a fastener on one of the minor panels to the third attached fastener of the lower sheet with the minor panel extending over and covering a portion of the upper sheet ; ( i ) detaching one of the plurality of minor panels from the lower sheet while retaining the upper and lower sheets on the mattress ; and ( j ) releasably securing the fastener of another of the plurality of minor panels to the third fastener of the lower sheet so that the minor panel extends over and covers the upper sheet , thereby extending the time between changes of the upper and lower sheet components of the bed sheet set . while the instant invention has been shown and described in accordance with various preferred and practical embodiments thereof , it is recognized that departures from the instant disclosure are contemplated within the spirit and scope of the present invention which , therefore , should not be limited except as defined in the following claims as interpreted under the doctrine of equivalents .